On Mon, Aug 22, 2016 at 9:53 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> When running with a local patch which moves the '_stext' symbol to the
> very beginning of the kernel text area, I got the following panic with
> CONFIG_HARDENED_USERCOPY:
>
> usercopy: kernel memory exposure attempt detected from ffff88103dfff000 (<linear kernel text>) (4096 bytes)
> ------------[ cut here ]------------
> kernel BUG at mm/usercopy.c:79!
> invalid opcode: 0000 [#1] SMP
> ...
> CPU: 0 PID: 4800 Comm: cp Not tainted 4.8.0-rc3.after+ #1
> Hardware name: Dell Inc. PowerEdge R720/0X3D66, BIOS 2.5.4 01/22/2016
> task: ffff880817444140 task.stack: ffff880816274000
> RIP: 0010:[<ffffffff8121c796>] __check_object_size+0x76/0x413
> RSP: 0018:ffff880816277c40 EFLAGS: 00010246
> RAX: 000000000000006b RBX: ffff88103dfff000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffff88081f80dfa8 RDI: ffff88081f80dfa8
> RBP: ffff880816277c90 R08: 000000000000054c R09: 0000000000000000
> R10: 0000000000000005 R11: 0000000000000006 R12: 0000000000001000
> R13: ffff88103e000000 R14: ffff88103dffffff R15: 0000000000000001
> FS: 00007fb9d1750800(0000) GS:ffff88081f800000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000021d2000 CR3: 000000081a08f000 CR4: 00000000001406f0
> Stack:
> ffff880816277cc8 0000000000010000 000000043de07000 0000000000000000
> 0000000000001000 ffff880816277e60 0000000000001000 ffff880816277e28
> 000000000000c000 0000000000001000 ffff880816277ce8 ffffffff8136c3a6
> Call Trace:
> [<ffffffff8136c3a6>] copy_page_to_iter_iovec+0xa6/0x1c0
> [<ffffffff8136e766>] copy_page_to_iter+0x16/0x90
> [<ffffffff811970e3>] generic_file_read_iter+0x3e3/0x7c0
> [<ffffffffa06a738d>] ? xfs_file_buffered_aio_write+0xad/0x260 [xfs]
> [<ffffffff816e6262>] ? down_read+0x12/0x40
> [<ffffffffa06a61b1>] xfs_file_buffered_aio_read+0x51/0xc0 [xfs]
> [<ffffffffa06a6692>] xfs_file_read_iter+0x62/0xb0 [xfs]
> [<ffffffff812224cf>] __vfs_read+0xdf/0x130
> [<ffffffff81222c9e>] vfs_read+0x8e/0x140
> [<ffffffff81224195>] SyS_read+0x55/0xc0
> [<ffffffff81003a47>] do_syscall_64+0x67/0x160
> [<ffffffff816e8421>] entry_SYSCALL64_slow_path+0x25/0x25
> RIP: 0033:[<00007fb9d0c33c00>] 0x7fb9d0c33c00
> RSP: 002b:00007ffc9c262f28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: fffffffffff8ffff RCX: 00007fb9d0c33c00
> RDX: 0000000000010000 RSI: 00000000021c3000 RDI: 0000000000000004
> RBP: 00000000021c3000 R08: 0000000000000000 R09: 00007ffc9c264d6c
> R10: 00007ffc9c262c50 R11: 0000000000000246 R12: 0000000000010000
> R13: 00007ffc9c2630b0 R14: 0000000000000004 R15: 0000000000010000
> Code: 81 48 0f 44 d0 48 c7 c6 90 4d a3 81 48 c7 c0 bb b3 a2 81 48 0f 44 f0 4d 89 e1 48 89 d9 48 c7 c7 68 16 a3 81 31 c0 e8 f4 57 f7 ff <0f> 0b 48 8d 90 00 40 00 00 48 39 d3 0f 83 22 01 00 00 48 39 c3
> RIP [<ffffffff8121c796>] __check_object_size+0x76/0x413
> RSP <ffff880816277c40>
>
> The checked object's range [ffff88103dfff000, ffff88103e000000) is
> valid, so there shouldn't have been a BUG. The hardened usercopy code
> got confused because the range's ending address is the same as the
> kernel's text starting address at 0xffff88103e000000. The overlap check
> is slightly off.
>
> Fixes: f5509cc18daa ("mm: Hardened usercopy")
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Ah! Excellent! I bet this is exactly the same thing that Geert saw.
Both are the start of kernel memory and would be intermittent
depending on allocation ordering.
Thanks! I'll get this applied.
-Kees
> ---
> mm/usercopy.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 8ebae91..6b1c20f 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -83,7 +83,7 @@ static bool overlaps(const void *ptr, unsigned long n, unsigned long low,
> unsigned long check_high = check_low + n;
>
> /* Does not overlap if entirely above or entirely below. */
> - if (check_low >= high || check_high < low)
> + if (check_low >= high || check_high <= low)
> return false;
>
> return true;
> --
> 2.7.4
>
--
Kees Cook
Nexus Security
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