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patch 9.0.2121: [security]: use-after-free in ex_substitute

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Problem:  [security]: use-after-free in ex_substitute
Solution: always allocate memory

closes: #13552

A recursive :substitute command could cause a heap-use-after free in Vim
(CVE-2023-48706).

The whole reproducible test is a bit tricky, I can only reproduce this
reliably when no previous substitution command has been used yet
(which is the reason, the test needs to run as first one in the
test_substitute.vim file) and as a combination of the `:~` command
together with a :s command that contains the special substitution atom `~\=`
which will make use of a sub-replace special atom and calls a vim script
function.

There was a comment in the existing :s code, that already makes the
`sub` variable allocate memory so that a recursive :s call won't be able
to cause any issues here, so this was known as a potential problem
already.  But for the current test-case that one does not work, because
the substitution does not start with `\=` but with `~\=` (and since
there does not yet exist a previous substitution atom, Vim will simply
increment the `sub` pointer (which then was not allocated dynamically)
and later one happily use a sub-replace special expression (which could
then free the `sub` var).

The following commit fixes this, by making the sub var always using
allocated memory, which also means we need to free the pointer whenever
we leave the function. Since sub is now always an allocated variable,
we also do no longer need the sub_copy variable anymore, since this one
was used to indicated when sub pointed to allocated memory (and had
therefore to be freed on exit) and when not.

Github Security Advisory:
https://github.com/vim/vim/security/advisories/GHSA-c8qm-x72m-q53q

Signed-off-by: Christian Brabandt <cb@256bit.org>
This commit is contained in:
Christian Brabandt
2023-11-22 21:26:41 +01:00
parent 8750e3cf81
commit 26c11c5688
3 changed files with 85 additions and 15 deletions
Download Patch File Download Diff File Expand all files Collapse all files

50
src/ex_cmds.c
View File

@ -3737,13 +3737,13 @@ ex_substitute(exarg_T *eap)
int save_do_all; // remember user specified 'g' flag
int save_do_ask; // remember user specified 'c' flag
char_u *pat = NULL, *sub = NULL; // init for GCC
char_u *sub_copy = NULL;
int delimiter;
int sublen;
int got_quit = FALSE;
int got_match = FALSE;
int which_pat;
char_u *cmd;
char_u *p;
int save_State;
linenr_T first_line = 0; // first changed line
linenr_T last_line= 0; // below last changed line AFTER the
@ -3827,8 +3827,12 @@ ex_substitute(exarg_T *eap)
* Small incompatibility: vi sees '\n' as end of the command, but in
* Vim we want to use '\n' to find/substitute a NUL.
*/
sub = cmd; // remember the start of the substitution
p = cmd; // remember the start of the substitution
cmd = skip_substitute(cmd, delimiter);
sub = vim_strsave(p);
if (sub == NULL)
// out of memory
return;
if (!eap->skip)
{
@ -3839,14 +3843,22 @@ ex_substitute(exarg_T *eap)
if (old_sub == NULL) // there is no previous command
{
emsg(_(e_no_previous_substitute_regular_expression));
vim_free(sub);
return;
}
sub = old_sub;
vim_free(sub);
sub = vim_strsave(old_sub);
if (sub == NULL)
// out of memory
return;
}
else
{
vim_free(old_sub);
old_sub = vim_strsave(sub);
if (old_sub == NULL)
// out of memory
return;
}
}
}
@ -3858,7 +3870,7 @@ ex_substitute(exarg_T *eap)
return;
}
pat = NULL; // search_regcomp() will use previous pattern
sub = old_sub;
sub = vim_strsave(old_sub);
// Vi compatibility quirk: repeating with ":s" keeps the cursor in the
// last column after using "$".
@ -3877,7 +3889,10 @@ ex_substitute(exarg_T *eap)
linenr_T joined_lines_count;
if (eap->skip)
{
vim_free(sub);
return;
}
curwin->w_cursor.lnum = eap->line1;
if (*cmd == 'l')
eap->flags = EXFLAG_LIST;
@ -3904,6 +3919,7 @@ ex_substitute(exarg_T *eap)
save_re_pat(RE_SUBST, pat, magic_isset());
// put pattern in history
add_to_history(HIST_SEARCH, pat, TRUE, NUL);
vim_free(sub);
return;
}
@ -3991,6 +4007,7 @@ ex_substitute(exarg_T *eap)
if (i <= 0 && !eap->skip && subflags.do_error)
{
emsg(_(e_positive_count_required));
vim_free(sub);
return;
}
else if (i >= INT_MAX)
@ -3998,6 +4015,7 @@ ex_substitute(exarg_T *eap)
char buf[20];
vim_snprintf(buf, sizeof(buf), "%ld", i);
semsg(_(e_val_too_large), buf);
vim_free(sub);
return;
}
eap->line1 = eap->line2;
@ -4016,17 +4034,22 @@ ex_substitute(exarg_T *eap)
if (eap->nextcmd == NULL)
{
semsg(_(e_trailing_characters_str), cmd);
vim_free(sub);
return;
}
}
if (eap->skip) // not executing commands, only parsing
{
vim_free(sub);
return;
}
if (!subflags.do_count && !curbuf->b_p_ma)
{
// Substitution is not allowed in non-'modifiable' buffer
emsg(_(e_cannot_make_changes_modifiable_is_off));
vim_free(sub);
return;
}
@ -4034,6 +4057,7 @@ ex_substitute(exarg_T *eap)
{
if (subflags.do_error)
emsg(_(e_invalid_command));
vim_free(sub);
return;
}
@ -4054,20 +4078,20 @@ ex_substitute(exarg_T *eap)
*/
if (sub[0] == '\\' && sub[1] == '=')
{
sub = vim_strsave(sub);
if (sub == NULL)
p = vim_strsave(sub);
vim_free(sub);
if (p == NULL)
return;
sub_copy = sub;
sub = p;
}
else
{
char_u *newsub = regtilde(sub, magic_isset());
p = regtilde(sub, magic_isset());
if (newsub != sub)
if (p != sub)
{
// newsub was allocated, free it later.
sub_copy = newsub;
sub = newsub;
vim_free(sub);
sub = p;
}
}
@ -4965,7 +4989,7 @@ outofmem:
#endif
vim_regfree(regmatch.regprog);
vim_free(sub_copy);
vim_free(sub);
// Restore the flag values, they can be used for ":&&".
subflags.do_all = save_do_all;