[PATCH 1/9] commit-graph: handle overflow in chunk_size checks

We check the size of chunks with fixed records by multiplying the width of each record by the number of commits in the file. Like:

  if (chunk_size != g->num_commits * GRAPH_DATA_WIDTH)

If this multiplication overflows, we may not notice a chunk is too small (which could later lead to out-of-bound reads).

In the current code this is only possible for the CDAT chunk, but the reasons are quite subtle. We compute g->num_commits by dividing the size of the OIDL chunk by the hash length (since it consists of a bunch of hashes). So we know that any size_t multiplication that uses a value smaller than the hash length cannot overflow. And the CDAT records are the only ones that are larger (the others are just 4-byte records). So it's worth fixing all of these, to make it clear that they're not subject to overflow (without having to reason about seemingly unrelated code).

The obvious thing to do is add an st_mult(), like:

  if (chunk_size != st_mult(g->num_commits, GRAPH_DATA_WIDTH))

And that certainly works, but it has one downside: if we detect an overflow, we'll immediately die(). But the commit graph is an optional file; if we run into other problems loading it, we'll generally return an error and fall back to accessing the full objects. Using st_mult() means a malformed file will abort the whole process.

So instead, we can do a division like this:

  if (chunk_size / GRAPH_DATA_WIDTH != g->num_commits)

where there's no possibility of overflow. We do lose a little bit of precision; due to integer division truncation we'd allow up to an extra GRAPH_DATA_WIDTH-1 bytes of data in the chunk. That's OK. Our main goal here is making sure we don't have too _few_ bytes, which would cause an out-of-bounds read (we could actually replace our "!=" with "<", but I think it's worth being a little pedantic, as a large mismatch could be a sign of other problems).

I didn't add a test here. We'd need to generate a very large graph file in order to get g->num_commits large enough to cause an overflow. And a later patch in this series will use this same division technique in a way that is much easier to trigger in the tests.

Signed-off-by: Jeff King <peff@peff.net>
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There are still several st_mult() calls within commit-graph.c, unrelated
to my jk/chunk-bounds series. I suspect they're all redundant, as the
chunk-size checks give a stricter bound. But checking and removing them
is a separate topic.

I think the midx code has similar st_mult() problems, but it's quite happy to die() on any error already. So I've left auditing that for another day.

Another alternative to the division is introducing an st_mult() variant like:

  if (!st_mult(&out, &a, &b))
        return error(...);

We may want to go there in the long run as part of lib-ification, but I didn't want to get into it for this series.

 commit-graph.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)