[PATCH 5/5] patch-delta: handle truncated copy parameters
- From
Jeff King <peff@peff.net>
- Date
- Aug 30, 2018, 07:12 UTC
- Message-ID
- <20180830071252.GE15420@sigill.intra.peff.net>
- In-Reply-To
- <20180830070548.GA15081@sigill.intra.peff.net>
When we see a delta command instructing us to copy bytes from the base, we have to read the offset and size from the delta stream. We do this without checking whether we're at the end of the stream, meaning we may read past the end of the buffer.
In practice this isn't exploitable in any interesting way because:
1. Deltas are always in packfiles, so we have at least a
20-byte trailer that we'll end up reading. 2. The worst case is that we try to perform a nonsense
copy from the base object into the result, based on
whatever was in the pack stream next. In most cases
this will simply fail due to our bounds-checks against
the base or the result. But even if you carefully constructed a pack stream for
which it succeeds, it wouldn't perform any delta
operation that you couldn't have simply included in a
non-broken form.But obviously it's poor form to read past the end of the buffer we've been given. Unfortunately there's no easy way to do a single length check, since the number of bytes we need depends on the number of bits set in the initial command byte. So we'll just check each byte as we parse. We can hide the complexity in a macro; it's ugly, but not as ugly as writing out each individual conditional.
Signed-off-by: Jeff King <peff@peff.net> --- patch-delta.c | 21 ++++++++++++++------- t/t5303-pack-corruption-resilience.sh | 2 +- 2 files changed, 15 insertions(+), 8 deletions(-)
diff --git a/patch-delta.c b/patch-delta.c index 283fb4b759..b5c8594db6 100644 --- a/patch-delta.c +++ b/patch-delta.c @@ -40,13 +40,20 @@ void *patch_delta(const void *src_buf, unsigned long src_size, cmd = *data++; if (cmd & 0x80) { unsigned long cp_off = 0, cp_size = 0; - if (cmd & 0x01) cp_off = *data++; - if (cmd & 0x02) cp_off |= (*data++ << 8); - if (cmd & 0x04) cp_off |= (*data++ << 16); - if (cmd & 0x08) cp_off |= ((unsigned) *data++ << 24); - if (cmd & 0x10) cp_size = *data++; - if (cmd & 0x20) cp_size |= (*data++ << 8); - if (cmd & 0x40) cp_size |= (*data++ << 16); +#define PARSE_CP_PARAM(bit, var, shift) do { \ + if (cmd & (bit)) { \ + if (data >= top) \ + goto bad_length; \ + var |= ((unsigned) *data++ << (shift)); \ + } } while (0) + PARSE_CP_PARAM(0x01, cp_off, 0); + PARSE_CP_PARAM(0x02, cp_off, 8); + PARSE_CP_PARAM(0x04, cp_off, 16); + PARSE_CP_PARAM(0x08, cp_off, 24); + PARSE_CP_PARAM(0x10, cp_size, 0); + PARSE_CP_PARAM(0x20, cp_size, 8); + PARSE_CP_PARAM(0x40, cp_size, 16); +#undef PARSE_CP_PARAM if (cp_size == 0) cp_size = 0x10000; if (unsigned_add_overflows(cp_off, cp_size) || cp_off + cp_size > src_size || diff --git a/t/t5303-pack-corruption-resilience.sh b/t/t5303-pack-corruption-resilience.sh index 41dc947d3f..b68bbeedcc 100755 --- a/t/t5303-pack-corruption-resilience.sh +++ b/t/t5303-pack-corruption-resilience.sh @@ -364,7 +364,7 @@ test_expect_success \ # # Note that the literal byte is necessary to get past the uninteresting minimum # delta size check. -test_expect_failure \ +test_expect_success \ 'apply delta with truncated copy parameters' \ 'printf "\5\5\1X\221" > truncated_copy_delta && echo base >base &&
-- 2.19.0.rc1.539.g3876d0831e