[Bug libc/17829] New: Incorrect handling of precision specifier in printf family

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[Bug libc/17829] New: Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

            Bug ID: 17829
           Summary: Incorrect handling of precision specifier in printf
                    family
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: critical
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: nfxjfg at googlemail dot com
                CC: drepper.fsp at gmail dot com

The following program shows no output:

#include <stdio.h>
#include <limits.h>

int main(int argc, char **argv)
{
    printf("%.*s\n", INT_MAX, "hi");
    return 0;
}

The precision given is INT_MAX; this should turn the precision specifier into a
no-OP, equivalent to 'printf("%s\n", "hi");'. Making the precision value
somewhat lower seems to make it work.

snprintf() seems to have a similar issue, but the failure seems to start with
even lower precision values.

Other libcs handle this correctly (I tested mingw-w64 and musl).

Using glibc 2.19-13 on Debian 32 bit x86.

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[Bug libc/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

nfxjfg at googlemail dot com changed:

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                 CC|                            |nfxjfg at googlemail dot com

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

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          Component|libc                        |stdio

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |fweimer at redhat dot com

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |codonell at redhat dot com

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
Carlos, do you remember what the “32” in stdio-common/vfprintf.c guards
against?  (You helped to fix some overflow-related issues in this area.)

   1574       if (prec > width
   1575           && prec > sizeof (work_buffer) / sizeof (work_buffer[0]) -
32)
   1576         {
   1577           if (__glibc_unlikely (prec >= INT_MAX / sizeof (CHAR_T) -
32))
   1578             {
   1579               __set_errno (EOVERFLOW);
   1580               done = -1;
   1581               goto all_done;
   1582             }
   1583           size_t needed = ((size_t) prec + 32) * sizeof (CHAR_T);

I'm a bit at a loss here.  Certainly, this use is not recommended because
printf will allocate tons of memory as part of the format processing.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Carlos O'Donell <carlos at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |carlos at redhat dot com

--- Comment #2 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Florian Weimer from comment #1)

> Carlos, do you remember what the “32” in stdio-common/vfprintf.c guards
> against?  (You helped to fix some overflow-related issues in this area.)
>
>    1574       if (prec > width
>    1575           && prec > sizeof (work_buffer) / sizeof (work_buffer[0]) -
> 32)
>    1576         {
>    1577           if (__glibc_unlikely (prec >= INT_MAX / sizeof (CHAR_T) -
> 32))
>    1578             {
>    1579               __set_errno (EOVERFLOW);
>    1580               done = -1;
>    1581               goto all_done;
>    1582             }
>    1583           size_t needed = ((size_t) prec + 32) * sizeof (CHAR_T);
>
> I'm a bit at a loss here.  Certainly, this use is not recommended because
> printf will allocate tons of memory as part of the format processing.

The +32 is an arbitrarily selected value to make the buffer large enough to be
OK for the largest precision we need. It is an artifact of sloppy accounting
for how much would be needed. The correct fix is to be more precise in
computing what we need.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #3 from nfxjfg at googlemail dot com ---
>Certainly, this use is not recommended because printf will allocate tons of memory as part of the format processing.

There's literally no reason why it'd need to allocate memory of the size of the
maximum _possible_ length of the string. In fact, I'd argue printf doesn't need
to do unbounded memory allocations at all.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

--- Comment #4 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to nfxjfg from comment #3)
> >Certainly, this use is not recommended because printf will allocate tons of memory as part of the format processing.
>
> There's literally no reason why it'd need to allocate memory of the size of
> the maximum _possible_ length of the string. In fact, I'd argue printf
> doesn't need to do unbounded memory allocations at all.

But it's what the code does today.  It could be implemented differently, sure,
but until someone writes the code, submits it, and gets it through review, it's
how things are.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Alexander Cherepanov <cherepan at mccme dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |cherepan at mccme dot ru

--- Comment #5 from Alexander Cherepanov <cherepan at mccme dot ru> ---
To add to two already mentioned problems (unnecessary malloc and fail for
INT_MAX):

C11 seems not to specify any limit for a precision and values larger than
INT_MAX could be useful. The variable for precision is declared as int[1] and
extracted from a format string as int with overflow control[2]. A larger type
like size_t and something like saturated processing of the number is a better
fit.

[1]
https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf.c;h=ae0145295479d0da11ae6bd496bd7039546419de;hb=HEAD#l1359
[2]
https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/printf-parse.h;h=e9903549997b113c5a38be3c2a463c2d83954c51;hb=HEAD#l68

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #6 from Alexander Cherepanov <cherepan at mccme dot ru> ---
Created attachment 8862
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8862&action=edit
Fix computations for max precision in vfprintf

While the full fix requires some work allowing INT_MAX on 64-bit platforms
seems easy.

The computation of the amount of the required memory is done in size_t but the
check uses INT_MAX for some reason. Replacing it with SIZE_MAX looks like a
right thing.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #7 from Alexander Cherepanov <cherepan at mccme dot ru> ---
1. The same problem with the width. Consider the program:

#include <stdio.h>
#include <errno.h>

int main()
{
   int res =  printf("%2147483647d", 1);
   fprintf(stderr, "res = %d, errno = %d\n", res, errno);
}

printf should succeed but it fails: "res = -1, errno = 75". There are
two problematic checks similar to the one for precision:

https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf.c;h=6829d4dc8e7fe7c066a06f1857ee926e0f48c379;hb=HEAD#l1459
https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf.c;h=6829d4dc8e7fe7c066a06f1857ee926e0f48c379;hb=HEAD#l1491

2. When the width is taken from an argument via asterisk, INT_MIN is not
handled properly -- signed integer overflow:

1451         /* Negative width means left justified.  */
1452         if (width < 0)
1453           {
1454             width = -width;
1455             pad = L_(' ');
1456             left = 1;
1457           }

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Vincent Lefèvre <vincent-srcware at vinc17 dot net> changed:

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                 CC|                            |vincent-srcware at vinc17 dot net

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #8 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
Similarly, assuming the defect in the C standard mentioned in bug 21360, the
following program:

#include <stdio.h>

int main(void)
{
  long n = -1;

  snprintf (NULL, 0, "%100000000000s%ln", "", &n);
  printf ("%ld\n", n);
  return 0;
}

outputs -1 instead of 100000000000 on x86_64 (64-bit long's).

Note: This may be regarded as being above the environmental limits, but nothing
seems to be documented about them, and it would be better to get the expected
result anyway.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #9 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
(In reply to Vincent Lefèvre from comment #8)
[...]
> outputs -1 instead of 100000000000 on x86_64 (64-bit long's).

Actually, I'm not sure of what is expected in case of overflow on the return
value. I've just asked in the Austin Group mailing-list:
https://www.mail-archive.com/austin-group-l@.../msg01038.html

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #10 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Vincent Lefèvre from comment #9)
> (In reply to Vincent Lefèvre from comment #8)
> [...]
> > outputs -1 instead of 100000000000 on x86_64 (64-bit long's).
>
> Actually, I'm not sure of what is expected in case of overflow on the return
> value. I've just asked in the Austin Group mailing-list:
> https://www.mail-archive.com/austin-group-l@.../msg01038.html

See bug 14771 comment 4.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #11 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
(In reply to Florian Weimer from comment #10)
> See bug 14771 comment 4.

This is a different issue. I'm not talking about the size argument, but the 'n'
format specifier.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #12 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Vincent Lefèvre from comment #11)
> (In reply to Florian Weimer from comment #10)
> > See bug 14771 comment 4.
>
> This is a different issue. I'm not talking about the size argument, but the
> 'n' format specifier.

They are related.  I'm not sure if it is reasonable to expect that if snprintf
fails with EOVERFLOW, %n output has been written.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #13 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
(In reply to Florian Weimer from comment #12)
> (In reply to Vincent Lefèvre from comment #11)
> > (In reply to Florian Weimer from comment #10)
> > > See bug 14771 comment 4.
> >
> > This is a different issue. I'm not talking about the size argument, but the
> > 'n' format specifier.
>
> They are related.

Bug 14771 comment 4 was about a conflict between ISO C and POSIX, and doesn't
give a hint here.

> I'm not sure if it is reasonable to expect that if snprintf fails with
> EOVERFLOW, %n output has been written.

There is no %n output. %n allows one to get information about the length of
some sequence of characters. According to bug 21360, one should get a
"meaningful" value, even for something that would *never* be output. So, if one
follows this idea, it could be reasonable to expect a meaningful value if int
is too small for the return value, e.g. to overcome the limitation of the int
type.

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

glaubitz at physik dot fu-berlin.de
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https://sourceware.org/bugzilla/show_bug.cgi?id=17829

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://sourceware.org/bugz
                   |                            |illa/show_bug.cgi?id=24988

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[Bug stdio/17829] Incorrect handling of precision specifier in printf family

Sourceware - glibc-bugs mailing list
In reply to this post by glaubitz at physik dot fu-berlin.de
https://sourceware.org/bugzilla/show_bug.cgi?id=17829

--- Comment #14 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Joseph Myers <[hidden email]>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6caddd34bd7ffb5ac4f36c8e036eee100c2cc535

commit 6caddd34bd7ffb5ac4f36c8e036eee100c2cc535
Author: Joseph Myers <[hidden email]>
Date:   Tue Jul 7 14:54:12 2020 +0000

    Remove most vfprintf width/precision-dependent allocations (bug 14231, bug
26211).

    The vfprintf implementation (used for all printf-family functions)
    contains complicated logic to allocate internal buffers of a size
    depending on the width and precision used for a format, using either
    malloc or alloca depending on that size, and with consequent checks
    for size overflow and allocation failure.

    As noted in bug 26211, the version of that logic used when '$' plus
    argument number formats are in use is missing the overflow checks,
    which can result in segfaults (quite possibly exploitable, I didn't
    try to work that out) when the width or precision is in the range
    0x7fffffe0 through 0x7fffffff (maybe smaller values as well in the
    wprintf case on 32-bit systems, when the multiplication by sizeof
    (CHAR_T) can overflow).

    All that complicated logic in fact appears to be useless.  As far as I
    can tell, there has been no need (outside the floating-point printf
    code, which does its own allocations) for allocations depending on
    width or precision since commit
    3e95f6602b226e0de06aaff686dc47b282d7cc16 ("Remove limitation on size
    of precision for integers", Sun Sep 12 21:23:32 1999 +0000).  Thus,
    this patch removes that logic completely, thereby fixing both problems
    with excessive allocations for large width and precision for
    non-floating-point formats, and the problem with missing overflow
    checks with such allocations.  Note that this does have the
    consequence that width and precision up to INT_MAX are now allowed
    where previously INT_MAX / sizeof (CHAR_T) - EXTSIZ or more would have
    been rejected, so could potentially expose any other overflows where
    the value would previously have been rejected by those removed checks.

    I believe this completely fixes bugs 14231 and 26211.

    Excessive allocations are still possible in the floating-point case
    (bug 21127), as are other integer or buffer overflows (see bug 26201).
    This does not address the cases where a precision larger than INT_MAX
    (embedded in the format string) would be meaningful without printf's
    return value overflowing (when it's used with a string format, or %g
    without the '#' flag, so the actual output will be much smaller), as
    mentioned in bug 17829 comment 8; using size_t internally for
    precision to handle that case would be complicated by struct
    printf_info being a public ABI.  Nor does it address the matter of an
    INT_MIN width being negated (bug 17829 comment 7; the same logic
    appears a second time in the file as well, in the form of multiplying
    by -1).  There may be other sources of memory allocations with malloc
    in printf functions as well (bug 24988, bug 16060).  From inspection,
    I think there are also integer overflows in two copies of "if ((width
    -= len) < 0)" logic (where width is int, len is size_t and a very long
    string could result in spurious padding being output on a 32-bit
    system before printf overflows the count of output characters).

    Tested for x86-64 and x86.

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