Re: [PATCH 2/4] crypto: dh - don't permit 'p' to be 0

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On 11/02/2017 12:25 AM, Eric Biggers wrote:
From: Eric Biggers <ebiggers@xxxxxxxxxx>

If 'p' is 0 for the software Diffie-Hellman implementation, then
dh_max_size() returns 0.  In the case of KEYCTL_DH_COMPUTE, this causes
ZERO_SIZE_POINTER to be passed to sg_init_one(), which with
CONFIG_DEBUG_SG=y triggers the 'BUG_ON(!virt_addr_valid(buf));' in
sg_set_buf().

Fix this by making crypto_dh_decode_key() reject 0 for 'p'.  p=0 makes
no sense for any DH implementation because 'p' is supposed to be a prime
number.  Moreover, 'mod 0' is not mathematically defined.

Bug report:

     kernel BUG at ./include/linux/scatterlist.h:140!
     invalid opcode: 0000 [#1] SMP KASAN
     CPU: 0 PID: 27112 Comm: syz-executor2 Not tainted 4.14.0-rc7-00010-gf5dbb5d0ce32-dirty #7
     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.3-20171021_125229-anatol 04/01/2014
     task: ffff88006caac0c0 task.stack: ffff88006c7c8000
     RIP: 0010:sg_set_buf include/linux/scatterlist.h:140 [inline]
     RIP: 0010:sg_init_one+0x1b3/0x240 lib/scatterlist.c:156
     RSP: 0018:ffff88006c7cfb08 EFLAGS: 00010216
     RAX: 0000000000010000 RBX: ffff88006c7cfe30 RCX: 00000000000064ee
     RDX: ffffffff81cf64c3 RSI: ffffc90000d72000 RDI: ffffffff92e937e0
     RBP: ffff88006c7cfb30 R08: ffffed000d8f9fab R09: ffff88006c7cfd30
     R10: 0000000000000005 R11: ffffed000d8f9faa R12: ffff88006c7cfd30
     R13: 0000000000000000 R14: 0000000000000010 R15: ffff88006c7cfc50
     FS:  00007fce190fa700(0000) GS:ffff88003ea00000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 00007fffc6b33db8 CR3: 000000003cf64000 CR4: 00000000000006f0
     Call Trace:
      __keyctl_dh_compute+0xa95/0x19b0 security/keys/dh.c:360
      keyctl_dh_compute+0xac/0x100 security/keys/dh.c:434
      SYSC_keyctl security/keys/keyctl.c:1745 [inline]
      SyS_keyctl+0x72/0x2c0 security/keys/keyctl.c:1641
      entry_SYSCALL_64_fastpath+0x1f/0xbe
     RIP: 0033:0x4585c9
     RSP: 002b:00007fce190f9bd8 EFLAGS: 00000216 ORIG_RAX: 00000000000000fa
     RAX: ffffffffffffffda RBX: 0000000000738020 RCX: 00000000004585c9
     RDX: 000000002000d000 RSI: 0000000020000ff4 RDI: 0000000000000017
     RBP: 0000000000000046 R08: 0000000020008000 R09: 0000000000000000
     R10: 0000000000000000 R11: 0000000000000216 R12: 00007fff6e610cde
     R13: 00007fff6e610cdf R14: 00007fce190fa700 R15: 0000000000000000
     Code: 03 0f b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 04 84 d2 75 33 5b 45 89 6c 24 14 41 5c 41 5d 41 5e 41 5f 5d c3 e8 fd 8f 68 ff <0f> 0b e8 f6 8f 68 ff 0f 0b e8 ef 8f 68 ff 0f 0b e8 e8 8f 68 ff 20
     RIP: sg_set_buf include/linux/scatterlist.h:140 [inline] RSP: ffff88006c7cfb08
     RIP: sg_init_one+0x1b3/0x240 lib/scatterlist.c:156 RSP: ffff88006c7cfb08

Fixes: 802c7f1c84e4 ("crypto: dh - Add DH software implementation")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.8+
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Reviewed-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxxxxx>



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