diff options
authorNicolai Stange <nicstange@gmail.com>2016-03-22 13:12:38 +0100
committerHerbert Xu <herbert@gondor.apana.org.au>2016-04-05 20:35:48 +0800
commitcece762f6f3cb1c1687f467e98faef21b0096600 (patch)
parentea122be0b8f788b30e71ed5536fddc05f5ddff86 (diff)
lib/mpi: mpi_write_sgl(): fix out-of-bounds stack access
Within the copying loop in mpi_write_sgl(), we have if (lzeros) { mpi_limb_t *limb1 = (void *)p - sizeof(alimb); mpi_limb_t *limb2 = (void *)p - sizeof(alimb) + lzeros; *limb1 = *limb2; ... } where p points past the end of alimb2 which lives on the stack and contains the current limb in BE order. The purpose of the above is to shift the non-zero bytes of alimb2 to its beginning in memory, i.e. to skip its leading zero bytes. However, limb2 points somewhere into the middle of alimb2 and thus, reading *limb2 pulls in lzero bytes from somewhere. Indeed, KASAN splats: BUG: KASAN: stack-out-of-bounds in mpi_write_to_sgl+0x4e3/0x6f0 at addr ffff8800cb04f601 Read of size 8 by task systemd-udevd/391 page:ffffea00032c13c0 count:0 mapcount:0 mapping: (null) index:0x0 flags: 0x3fff8000000000() page dumped because: kasan: bad access detected CPU: 3 PID: 391 Comm: systemd-udevd Tainted: G B L 4.5.0-next-20160316+ #12 [...] Call Trace: [<ffffffff8194889e>] dump_stack+0xdc/0x15e [<ffffffff819487c2>] ? _atomic_dec_and_lock+0xa2/0xa2 [<ffffffff814892b5>] ? __dump_page+0x185/0x330 [<ffffffff8150ffd6>] kasan_report_error+0x5e6/0x8b0 [<ffffffff814724cd>] ? kzfree+0x2d/0x40 [<ffffffff819c5bce>] ? mpi_free_limb_space+0xe/0x20 [<ffffffff819c469e>] ? mpi_powm+0x37e/0x16f0 [<ffffffff815109f1>] kasan_report+0x71/0xa0 [<ffffffff819c0353>] ? mpi_write_to_sgl+0x4e3/0x6f0 [<ffffffff8150ed34>] __asan_load8+0x64/0x70 [<ffffffff819c0353>] mpi_write_to_sgl+0x4e3/0x6f0 [<ffffffff819bfe70>] ? mpi_set_buffer+0x620/0x620 [<ffffffff819c0e6f>] ? mpi_cmp+0xbf/0x180 [<ffffffff8186e282>] rsa_verify+0x202/0x260 What's more, since lzeros can be anything from 1 to sizeof(mpi_limb_t)-1, the above will cause unaligned accesses which is bad on non-x86 archs. Fix the issue, by preparing the starting point p for the upcoming copy operation instead of shifting the source memory, i.e. alimb2. Fixes: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers") Signed-off-by: Nicolai Stange <nicstange@gmail.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
1 files changed, 1 insertions, 5 deletions
diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 78ec4e1131b9..b05d3902d363 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -403,15 +403,11 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
#error please implement for this limb size.
if (lzeros) {
- mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
- mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
- + lzeros;
- *limb1 = *limb2;
y = lzeros;
lzeros = 0;
- p = p - sizeof(alimb);
+ p = p - sizeof(alimb) + y;
for (x = 0; x < sizeof(alimb) - y; x++) {
if (!buf_len) {