[PATCH] Fix use-after-free in glob when expanding ~user (bug 25414)

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[PATCH] Fix use-after-free in glob when expanding ~user (bug 25414)

Andreas Schwab
The value of `end_name' points into the value of `dirname', thus don't
deallocate the latter before the last use of the former.
---
 posix/glob.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/posix/glob.c b/posix/glob.c
index cba9cd1819..0708ed5ee4 100644
--- a/posix/glob.c
+++ b/posix/glob.c
@@ -827,31 +827,32 @@ __glob (const char *pattern, int flags, int (*errfunc) (const char *, int),
       {
  size_t home_len = strlen (p->pw_dir);
  size_t rest_len = end_name == NULL ? 0 : strlen (end_name);
- char *d;
+ char *d, *newp;
+ int use_alloca = glob_use_alloca (alloca_used,
+  home_len + rest_len + 1);
 
- if (__glibc_unlikely (malloc_dirname))
-  free (dirname);
- malloc_dirname = 0;
-
- if (glob_use_alloca (alloca_used, home_len + rest_len + 1))
-  dirname = alloca_account (home_len + rest_len + 1,
-    alloca_used);
+ if (use_alloca)
+  newp = alloca_account (home_len + rest_len + 1, alloca_used);
  else
   {
-    dirname = malloc (home_len + rest_len + 1);
-    if (dirname == NULL)
+    newp = malloc (home_len + rest_len + 1);
+    if (newp == NULL)
       {
  scratch_buffer_free (&pwtmpbuf);
  retval = GLOB_NOSPACE;
  goto out;
       }
-    malloc_dirname = 1;
   }
- d = mempcpy (dirname, p->pw_dir, home_len);
+ d = mempcpy (newp, p->pw_dir, home_len);
  if (end_name != NULL)
   d = mempcpy (d, end_name, rest_len);
  *d = '\0';
 
+ if (__glibc_unlikely (malloc_dirname))
+  free (dirname);
+ dirname = newp;
+ malloc_dirname = !use_alloca;
+
  dirlen = home_len + rest_len;
  dirname_modified = 1;
       }
--
2.25.1


--
Andreas Schwab, SUSE Labs, [hidden email]
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
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Re: [PATCH] Fix use-after-free in glob when expanding ~user (bug 25414)

Paul Eggert
On 2/19/20 8:50 AM, Andreas Schwab wrote:

> + int use_alloca = glob_use_alloca (alloca_used,
> +  home_len + rest_len + 1);

I suggest using 'bool' instead of 'int' here, since the value is boolean
and glob_use_alloca returns 'bool'. Although some of the nearby local
vars use 'int' to represent booleans, surely this is because the code
predates 'bool'; for new code it's better to use 'bool' for booleans.

Otherwise, looks good; thanks.