Re: [PATCH 02/20] packfile.c: prevent overflow in `load_idx()`

[Taylor Blau <me@ttaylorr.com>]

On Thu, Jul 13, 2023 at 10:24:53AM -0400, Taylor Blau wrote:
> On Thu, Jul 13, 2023 at 09:21:55AM +0100, Phillip Wood wrote:
> > p->crc_offset is a uint32_t so we're still prone to truncation here unless
> > we change the crc_offset member of struct packed_git to be a size_t. I
> > haven't checked if the other users of crc_offset would need adjusting if we
> > change its type.
>
> Thanks for spotting. Luckily, this should be a straightforward change:

Here's a replacement patch which changes the type of `crc_offset`. If
there end up being other review comments, I'll fold this into the next
round.

--- 8< ---
Subject: [PATCH] packfile.c: prevent overflow in `load_idx()`

Prevent an overflow when locating a pack's CRC offset when the number
of packed items is greater than 2^32-1/hashsz by guarding the
computation with an `st_mult()`.

Note that to avoid truncating the result, the `crc_offset` member must
itself become a `size_t`. The only usage of this variable (besides the
assignment in `load_idx()`) is in `read_v2_anomalous_offsets()` in the
index-pack code. There we use the `crc_offset` as a pointer offset, so
we are already equipped to handle the type change.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 object-store-ll.h | 2 +-
 packfile.c        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/object-store-ll.h b/object-store-ll.h
index e8f22cdb1b..26a3895c82 100644
--- a/object-store-ll.h
+++ b/object-store-ll.h
@@ -106,7 +106,7 @@ struct packed_git {
 	const void *index_data;
 	size_t index_size;
 	uint32_t num_objects;
-	uint32_t crc_offset;
+	size_t crc_offset;
 	struct oidset bad_objects;
 	int index_version;
 	time_t mtime;
diff --git a/packfile.c b/packfile.c
index 89220f0e03..70acf1694b 100644
--- a/packfile.c
+++ b/packfile.c
@@ -186,7 +186,7 @@ int load_idx(const char *path, const unsigned int hashsz, void *idx_map,
 		     */
 		    (sizeof(off_t) <= 4))
 			return error("pack too large for current definition of off_t in %s", path);
-		p->crc_offset = 8 + 4 * 256 + nr * hashsz;
+		p->crc_offset = st_add(8 + 4 * 256, st_mult(nr, hashsz));
 	}

 	p->index_version = version;
--
2.41.0.329.g0a1adfae833
--- >8 ---

Thanks,
Taylor