[PATCH] crypto: fix OOB read in pefile_digest_pe_contents

[Weiming Shi <bestswngs@gmail.com>]

pefile_digest_pe_contents() computes the trailing-data hash length as
pelen - (hashed_bytes + certs_size). A crafted PE can make the addition
exceed pelen, causing the unsigned subtraction to underflow to ~4 GiB.
This is passed to crypto_shash_update() which reads out of bounds and
panics on unmapped vmalloc guard pages.

 BUG: unable to handle page fault for address: ffffc900038d8000
 Oops: Oops: 0000 [#1] SMP KASAN NOPTI
 RIP: 0010:sha256_blocks_generic (lib/crypto/sha256.c:152)
 Call Trace:
  <TASK>
  __sha256_update (lib/crypto/sha256.c:208)
  crypto_sha256_update (crypto/sha256.c:142)
  verify_pefile_signature (crypto/asymmetric_keys/verify_pefile.c:436)
  kexec_kernel_verify_pe_sig (kernel/kexec_file.c:151)
  __do_sys_kexec_file_load (kernel/kexec_file.c:406)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  </TASK>
 Kernel panic - not syncing: Fatal exception

Validate that the addition does not overflow and the result does not
exceed pelen before the subtraction. Return -ELIBBAD on failure.

Fixes: af316fc442ef ("pefile: Digest the PE binary and compare to the PKCS#7 data")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 crypto/asymmetric_keys/verify_pefile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/crypto/asymmetric_keys/verify_pefile.c b/crypto/asymmetric_keys/verify_pefile.c
index 1f3b227ba7f2..cec99db14129 100644
--- a/crypto/asymmetric_keys/verify_pefile.c
+++ b/crypto/asymmetric_keys/verify_pefile.c
@@ -305,6 +305,8 @@ static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
 
 	if (pelen > hashed_bytes) {
 		tmp = hashed_bytes + ctx->certs_size;
+		if (tmp <= hashed_bytes || pelen < tmp)
+			return -ELIBBAD;
 		ret = crypto_shash_update(desc,
 					  pebuf + hashed_bytes,
 					  pelen - tmp);
-- 
2.43.0