From: Jeff King <peff@peff•net>
To: git@vger•kernel.org
Cc: Patrick Steinhardt <ps@pks•im>, correctmost <cmlists@sent•com>,
Taylor Blau <me@ttaylorr•com>
Subject: [PATCH v2 4/9] cache-tree: avoid strtol() on non-string buffer
Date: Tue, 18 Nov 2025 04:12:18 -0500 [thread overview]
Message-ID: <20251118091218.GD529192@coredump.intra.peff.net> (raw)
In-Reply-To: <20251118091127.GA4175601@coredump.intra.peff.net>
A cache-tree extension entry in the index looks like this:
<name> NUL <entry_nr> SPACE <subtree_nr> NEWLINE <binary_oid>
where the "_nr" items are human-readable base-10 ASCII. We parse them
with strtol(), even though we do not have a NUL-terminated string (we'd
generally have an mmap() of the on-disk index file). For a well-formed
entry, this is not a problem; strtol() will stop when it sees the
newline. But there are two problems:
1. A corrupted entry could omit the newline, causing us to read
further. You'd mostly get stopped by seeing non-digits in the oid
field (and if it is likewise truncated, there will still be 20 or
more bytes of the index checksum). So it's possible, though
unlikely, to read off the end of the mmap'd buffer. Of course a
malicious index file can fake the oid and the index checksum to all
(ASCII) 0's.
This is further complicated by the fact that mmap'd buffers tend to
be zero-padded up to the page boundary. So to run off the end, the
index size also has to be a multiple of the page size. This is also
unlikely, though you can construct a malicious index file that
matches this.
The security implications aren't too interesting. The index file is
a local file anyway (so you can't attack somebody by cloning, but
only if you convince them to operate in a .git directory you made,
at which point attacking .git/config is much easier). And it's just
a read overflow via strtol(), which is unlikely to buy you much
beyond a crash.
2. ASan has a strict_string_checks option, which tells it to make sure
that options to string functions (like strtol) have some eventual
NUL, without regard to what the function would actually do (like
stopping at a newline here). This option sometimes has false
positives, but it can point to sketchy areas (like this one) where
the input we use doesn't exhibit a problem, but different input
_could_ cause us to misbehave.
Let's fix it by just parsing the values ourselves with a helper function
that is careful not to go past the end of the buffer. There are a few
behavior changes here that should not matter:
- We do not consider overflow, as strtol() would. But nor did the
original code. However, we don't trust the value we get from the
on-disk file, and if it says to read 2^30 entries, we would notice
that we do not have that many and bail before reading off the end of
the buffer.
- Our helper does not skip past extra leading whitespace as strtol()
would, but according to gitformat-index(5) there should not be any.
- The original quit parsing at a newline or a NUL byte, but now we
insist on a newline (which is what the documentation says, and what
Git has always produced).
Since we are providing our own helper function, we can tweak the
interface a bit to make our lives easier. The original code does not use
strtol's "end" pointer to find the end of the parsed data, but rather
uses a separate loop to advance our "buf" pointer to the trailing
newline. We can instead provide a helper that advances "buf" as it
parses, letting us read strictly left-to-right through the buffer.
I didn't add a new test here. It's surprisingly difficult to construct
an index of exactly the right size due to the way we pad entries. But it
is easy to trigger the problem in existing tests when using ASan's
strict string checking, coupled with a recent change to use NO_MMAP with
ASan builds. So:
make SANITIZE=address
cd t
ASAN_OPTIONS=strict_string_checks=1 ./t0090-cache-tree.sh
triggers it reliably. Technically it is not deterministic because there
is ~8% chance (it's 1-(255/256)^20, or ^32 for sha256) that the trailing
checksum hash has a NUL byte in it. But we compute enough cache-trees in
the course of that script that we are very likely to hit the problem in
one of them.
We can look at making strict_string_checks the default for ASan builds,
but there are some other cases we'd want to fix first.
Reported-by: correctmost <cmlists@sent•com>
Signed-off-by: Jeff King <peff@peff•net>
---
cache-tree.c | 50 +++++++++++++++++++++++++++++++++++++-------------
1 file changed, 37 insertions(+), 13 deletions(-)
diff --git a/cache-tree.c b/cache-tree.c
index 2aba47060e..2d8947b518 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -548,12 +548,41 @@ void cache_tree_write(struct strbuf *sb, struct cache_tree *root)
trace2_region_leave("cache_tree", "write", the_repository);
}
+static int parse_int(const char **ptr, unsigned long *len_p, int *out)
+{
+ const char *s = *ptr;
+ unsigned long len = *len_p;
+ int ret = 0;
+ int sign = 1;
+
+ while (len && *s == '-') {
+ sign *= -1;
+ s++;
+ len--;
+ }
+
+ while (len) {
+ if (!isdigit(*s))
+ break;
+ ret *= 10;
+ ret += *s - '0';
+ s++;
+ len--;
+ }
+
+ if (s == *ptr)
+ return -1;
+
+ *ptr = s;
+ *len_p = len;
+ *out = sign * ret;
+ return 0;
+}
+
static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
{
const char *buf = *buffer;
unsigned long size = *size_p;
- const char *cp;
- char *ep;
struct cache_tree *it;
int i, subtree_nr;
const unsigned rawsz = the_hash_algo->rawsz;
@@ -569,19 +598,14 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
buf++; size--;
it = cache_tree();
- cp = buf;
- it->entry_count = strtol(cp, &ep, 10);
- if (cp == ep)
+ if (parse_int(&buf, &size, &it->entry_count) < 0)
goto free_return;
- cp = ep;
- subtree_nr = strtol(cp, &ep, 10);
- if (cp == ep)
+ if (!size || *buf != ' ')
goto free_return;
- while (size && *buf && *buf != '\n') {
- size--;
- buf++;
- }
- if (!size)
+ buf++; size--;
+ if (parse_int(&buf, &size, &subtree_nr) < 0)
+ goto free_return;
+ if (!size || *buf != '\n')
goto free_return;
buf++; size--;
if (0 <= it->entry_count) {
--
2.52.0.278.gadc6434dc3
next prev parent reply other threads:[~2025-11-18 9:12 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-11-12 7:55 [PATCH 0/9] asan bonanza Jeff King
2025-11-12 7:56 ` [PATCH 1/9] compat/mmap: mark unused argument in git_munmap() Jeff King
2025-11-12 8:01 ` [PATCH 2/9] pack-bitmap: handle name-hash lookups in incremental bitmaps Jeff King
2025-11-12 11:25 ` Patrick Steinhardt
2025-11-13 2:55 ` Taylor Blau
2025-11-18 8:59 ` Jeff King
2025-11-12 8:02 ` [PATCH 3/9] Makefile: turn on NO_MMAP when building with ASan Jeff King
2025-11-12 8:17 ` Collin Funk
2025-11-12 10:31 ` Jeff King
2025-11-12 20:06 ` Collin Funk
2025-11-12 11:26 ` Patrick Steinhardt
2025-11-13 3:12 ` Taylor Blau
2025-11-13 6:34 ` Patrick Steinhardt
2025-11-18 8:49 ` Jeff King
2025-11-13 16:30 ` Junio C Hamano
2025-11-14 7:00 ` Patrick Steinhardt
2025-11-15 2:13 ` Jeff King
2025-11-12 8:05 ` [PATCH 4/9] cache-tree: avoid strtol() on non-string buffer Jeff King
2025-11-12 11:26 ` Patrick Steinhardt
2025-11-13 3:09 ` Taylor Blau
2025-11-18 8:40 ` Jeff King
2025-11-18 8:38 ` Jeff King
2025-11-12 8:06 ` [PATCH 5/9] fsck: assert newline presence in fsck_ident() Jeff King
2025-11-12 8:06 ` [PATCH 6/9] fsck: avoid strcspn() " Jeff King
2025-11-12 8:06 ` [PATCH 7/9] fsck: remove redundant date timestamp check Jeff King
2025-11-12 8:10 ` [PATCH 8/9] fsck: avoid parse_timestamp() on buffer that isn't NUL-terminated Jeff King
2025-11-12 11:25 ` Patrick Steinhardt
2025-11-12 19:36 ` Junio C Hamano
2025-11-15 2:12 ` Jeff King
2025-11-12 8:10 ` [PATCH 9/9] t: enable ASan's strict_string_checks option Jeff King
2025-11-13 3:17 ` [PATCH 0/9] asan bonanza Taylor Blau
2025-11-18 9:11 ` [PATCH v2 " Jeff King
2025-11-18 9:11 ` [PATCH v2 1/9] compat/mmap: mark unused argument in git_munmap() Jeff King
2025-11-18 9:12 ` [PATCH v2 2/9] pack-bitmap: handle name-hash lookups in incremental bitmaps Jeff King
2025-11-18 9:12 ` [PATCH v2 3/9] Makefile: turn on NO_MMAP when building with ASan Jeff King
2025-11-18 9:12 ` Jeff King [this message]
2025-11-18 14:30 ` [PATCH v2 4/9] cache-tree: avoid strtol() on non-string buffer Phillip Wood
2025-11-23 6:19 ` Junio C Hamano
2025-11-23 15:51 ` Phillip Wood
2025-11-23 18:06 ` Junio C Hamano
2025-11-24 22:30 ` Jeff King
2025-11-24 23:09 ` Junio C Hamano
2025-11-26 15:09 ` Jeff King
2025-11-26 17:22 ` Junio C Hamano
2025-11-30 13:13 ` [PATCH 0/4] more robust functions for parsing int from buf Jeff King
2025-11-30 13:14 ` [PATCH 1/4] parse: prefer bool to int for boolean returns Jeff King
2025-12-04 11:23 ` Patrick Steinhardt
2025-11-30 13:15 ` [PATCH 2/4] parse: add functions for parsing from non-string buffers Jeff King
2025-11-30 13:46 ` my complaints with clar Jeff King
2025-12-01 14:16 ` Phillip Wood
2025-12-04 11:09 ` Patrick Steinhardt
2025-12-05 18:30 ` Jeff King
2025-12-04 11:23 ` [PATCH 2/4] parse: add functions for parsing from non-string buffers Patrick Steinhardt
2025-12-05 16:11 ` Phillip Wood
2026-01-20 20:54 ` Junio C Hamano
2026-01-21 5:27 ` Jeff King
2025-11-30 13:15 ` [PATCH 3/4] cache-tree: use parse_int_from_buf() Jeff King
2025-11-30 13:16 ` [PATCH 4/4] fsck: use parse_unsigned_from_buf() for parsing timestamp Jeff King
2025-11-18 9:12 ` [PATCH v2 5/9] fsck: assert newline presence in fsck_ident() Jeff King
2025-11-18 9:12 ` [PATCH v2 6/9] fsck: avoid strcspn() " Jeff King
2025-11-18 9:12 ` [PATCH v2 7/9] fsck: remove redundant date timestamp check Jeff King
2025-11-18 9:12 ` [PATCH v2 8/9] fsck: avoid parse_timestamp() on buffer that isn't NUL-terminated Jeff King
2025-11-18 9:12 ` [PATCH v2 9/9] t: enable ASan's strict_string_checks option Jeff King
2025-11-23 5:49 ` [PATCH v2 0/9] asan bonanza Junio C Hamano
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