aboutsummaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorJim Meyering <meyering@redhat.com>2012-04-23 20:06:35 (GMT)
committerJason A. Donenfeld <Jason@zx2c4.com>2012-09-30 17:30:07 (GMT)
commit7757d1b046ecb67b830151d20715c658867df1ec (patch)
treec7baaeded591a784482045dc1994e30a182e8019
parentui: output age in seconds if under 0 minues (diff)
downloadcgit-7757d1b046ecb67b830151d20715c658867df1ec.zip
cgit-7757d1b046ecb67b830151d20715c658867df1ec.tar.xz
do not write outside heap buffer
* parsing.c (substr): Handle tail < head. This started when I noticed some cgit segfaults on savannah.gnu.org. Finding the offending URL/commit and then constructing a stand-alone reproducer were far more time-consuming than writing the actual patch. The problem arises with a commit like this, in which the user name part of the "Author" field is empty: $ git log -1 commit 6f3f41d73393278f3ede68a2cb1e7a2a23fa3421 Author: <T at h.or> Date: Mon Apr 23 22:29:16 2012 +0200 Here's what happens: (this is due to buf=malloc(0); strncpy (buf, head, -1); where "head" may point to plenty of attacker-specified non-NUL bytes, so we can overwrite a zero-length heap buffer with arbitrary data) Invalid write of size 1 at 0x4A09361: strncpy (mc_replace_strmem.c:463) by 0x408977: substr (parsing.c:61) by 0x4089EF: parse_user (parsing.c:73) by 0x408D10: cgit_parse_commit (parsing.c:153) by 0x40A540: cgit_mk_refinfo (shared.c:171) by 0x40A581: cgit_refs_cb (shared.c:181) by 0x43DEB3: do_for_each_ref (refs.c:690) by 0x41075E: cgit_print_branches (ui-refs.c:191) by 0x416EF2: cgit_print_summary (ui-summary.c:56) by 0x40780A: summary_fn (cmd.c:120) by 0x40667A: process_request (cgit.c:544) by 0x404078: cache_process (cache.c:322) Address 0x4c718d0 is 0 bytes after a block of size 0 alloc'd at 0x4A0884D: malloc (vg_replace_malloc.c:263) by 0x455C85: xmalloc (wrapper.c:35) by 0x40894C: substr (parsing.c:60) by 0x4089EF: parse_user (parsing.c:73) by 0x408D10: cgit_parse_commit (parsing.c:153) by 0x40A540: cgit_mk_refinfo (shared.c:171) by 0x40A581: cgit_refs_cb (shared.c:181) by 0x43DEB3: do_for_each_ref (refs.c:690) by 0x41075E: cgit_print_branches (ui-refs.c:191) by 0x416EF2: cgit_print_summary (ui-summary.c:56) by 0x40780A: summary_fn (cmd.c:120) by 0x40667A: process_request (cgit.c:544) Invalid write of size 1 at 0x4A09400: strncpy (mc_replace_strmem.c:463) by 0x408977: substr (parsing.c:61) by 0x4089EF: parse_user (parsing.c:73) by 0x408D10: cgit_parse_commit (parsing.c:153) by 0x40A540: cgit_mk_refinfo (shared.c:171) by 0x40A581: cgit_refs_cb (shared.c:181) by 0x43DEB3: do_for_each_ref (refs.c:690) by 0x41075E: cgit_print_branches (ui-refs.c:191) by 0x416EF2: cgit_print_summary (ui-summary.c:56) by 0x40780A: summary_fn (cmd.c:120) by 0x40667A: process_request (cgit.c:544) by 0x404078: cache_process (cache.c:322) Address 0x4c7192b is not stack'd, malloc'd or (recently) free'd Invalid write of size 1 at 0x4A0940E: strncpy (mc_replace_strmem.c:463) by 0x408977: substr (parsing.c:61) by 0x4089EF: parse_user (parsing.c:73) by 0x408D10: cgit_parse_commit (parsing.c:153) by 0x40A540: cgit_mk_refinfo (shared.c:171) by 0x40A581: cgit_refs_cb (shared.c:181) by 0x43DEB3: do_for_each_ref (refs.c:690) by 0x41075E: cgit_print_branches (ui-refs.c:191) by 0x416EF2: cgit_print_summary (ui-summary.c:56) by 0x40780A: summary_fn (cmd.c:120) by 0x40667A: process_request (cgit.c:544) by 0x404078: cache_process (cache.c:322) Address 0x4c7192d is not stack'd, malloc'd or (recently) free'd Process terminating with default action of signal 11 (SIGSEGV) Access not within mapped region at address 0x502F000 at 0x4A09400: strncpy (mc_replace_strmem.c:463) by 0x408977: substr (parsing.c:61) by 0x4089EF: parse_user (parsing.c:73) by 0x408D10: cgit_parse_commit (parsing.c:153) by 0x40A540: cgit_mk_refinfo (shared.c:171) by 0x40A581: cgit_refs_cb (shared.c:181) by 0x43DEB3: do_for_each_ref (refs.c:690) by 0x41075E: cgit_print_branches (ui-refs.c:191) by 0x416EF2: cgit_print_summary (ui-summary.c:56) by 0x40780A: summary_fn (cmd.c:120) by 0x40667A: process_request (cgit.c:544) by 0x404078: cache_process (cache.c:322) This happens when tail - head == -1 here: (parsing.c) char *substr(const char *head, const char *tail) { char *buf; buf = xmalloc(tail - head + 1); strncpy(buf, head, tail - head); buf[tail - head] = '\0'; return buf; } char *parse_user(char *t, char **name, char **email, unsigned long *date) { char *p = t; int mode = 1; while (p && *p) { if (mode == 1 && *p == '<') { *name = substr(t, p - 1); t = p; mode++; } else if (mode == 1 && *p == '\n') { The fix is to handle the case of (tail < head) before calling xmalloc, thus avoiding passing an invalid value to xmalloc. And here's the reproducer: It was tricky to reproduce, because git prohibits use of an empty "name" in a commit ID. To construct the offending commit, I had to resort to using "git hash-object". git init -q foo && ( cd foo && echo a > j && git add . && git ci -q --author='au <T at h.or>' -m. . && h=$(git cat-file commit HEAD|sed 's/au //' \ |git hash-object -t commit -w --stdin) && git co -q -b test $h && git br -q -D master && git br -q -m test master) git clone -q --bare foo foo.git cat <<EOF > in repo.url=foo.git repo.path=foo.git EOF CGIT_CONFIG=in QUERY_STRING=url=foo.git valgrind ./cgit The valgrind output is what you see above. AFAICS, this is not exploitable thanks (ironically) to the use of strncpy. Since that -1 translates to SIZE_MAX and this is strncpy, not only does it copy whatever is in "head" (up to first NUL), but it also writes SIZE_MAX - strlen(head) NUL bytes into the destination buffer, and that latter is guaranteed to evoke a segfault. Since cgit is single-threaded, AFAICS, there is no way that the buffer clobbering can be turned into an exploit.
-rw-r--r--parsing.c2
1 files changed, 2 insertions, 0 deletions
diff --git a/parsing.c b/parsing.c
index 602e3de..1b2a551 100644
--- a/parsing.c
+++ b/parsing.c
@@ -56,6 +56,8 @@ char *substr(const char *head, const char *tail)
{
char *buf;
+ if (tail < head)
+ return xstrdup("");
buf = xmalloc(tail - head + 1);
strncpy(buf, head, tail - head);
buf[tail - head] = '\0';