Re: [PATCH] mm: fix overlap check in hardened usercopy

[Kees Cook]

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