demux_mf: improve format string processing

Before this commit, the user could specify a printf format string
which wasn't verified, and could result in:
- Undefined behavior due to missing or non-matching arguments.
- Buffer overflow due to untested result length.

The offending code was added at commit 103a9609 (2002, mplayer svn):
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@4566 b3059339-0415-0410-9bf9-f77b7e298cf2

It moved around but was not modified meaningfully until now.

Now we reject all conversion specifiers at the format except %%
and a simple subset of the valid specifiers. Also, we now use
snprintf to avoid buffer overflow.

The format string is provided by the user as part of mf:// URI.

Report and initial patch by Stefan Schiller.
Patch reviewed by @jeeb, @sfan5, Stefan Schiller.
This commit is contained in:
Avi Halachmi (:avih) 2021-04-04 14:11:15 +03:00
parent ef9596f78e
commit d0c530919d
1 changed files with 37 additions and 2 deletions

View File

@ -121,7 +121,8 @@ static mf_t *open_mf_pattern(void *talloc_ctx, struct demuxer *d, char *filename
goto exit_mf;
}
char *fname = talloc_size(mf, strlen(filename) + 32);
size_t fname_avail = strlen(filename) + 32;
char *fname = talloc_size(mf, fname_avail);
#if HAVE_GLOB
if (!strchr(filename, '%')) {
@ -148,10 +149,44 @@ static mf_t *open_mf_pattern(void *talloc_ctx, struct demuxer *d, char *filename
}
#endif
// We're using arbitrary user input as printf format with 1 int argument.
// Any format which uses exactly 1 int argument would be valid, but for
// simplicity we reject all conversion specifiers except %% and simple
// integer specifier: %[.][NUM]d where NUM is 1-3 digits (%.d is valid)
const char *f = filename;
int MAXDIGS = 3, nspec = 0, bad_spec = 0, c;
while (nspec < 2 && (c = *f++)) {
if (c != '%')
continue;
if (*f != '%') {
nspec++; // conversion specifier which isn't %%
if (*f == '.')
f++;
for (int ndig = 0; mp_isdigit(*f) && ndig < MAXDIGS; ndig++, f++)
/* no-op */;
if (*f != 'd') {
bad_spec++; // not int, or beyond our validation capacity
break;
}
}
// *f is '%' or 'd'
f++;
}
// nspec==0 (zero specifiers) is rejected because fname wouldn't advance.
if (bad_spec || nspec != 1) {
mp_err(log, "unsupported expr format: '%s'\n", filename);
goto exit_mf;
}
mp_info(log, "search expr: %s\n", filename);
while (error_count < 5) {
sprintf(fname, filename, count++);
if (snprintf(fname, fname_avail, filename, count++) >= fname_avail) {
mp_err(log, "format result too long: '%s'\n", filename);
goto exit_mf;
}
if (!mp_path_exists(fname)) {
error_count++;
mp_verbose(log, "file not found: '%s'\n", fname);