Re: [PATCH] mm/ksm: Fix kernel NULL pointer dereference at 0000000000000040

[David Hildenbrand]

On 14.04.20 08:20, Muchun Song wrote:
> The find_mergeable_vma can return NULL. In this case, it leads
> to crash when we access vma->vm_mm(which's offset is 0x40) in
> write_protect_page. And this case did happen on our server. The
> following calltrace is captured in kernel 4.19 with ksm enabled.
> So add a vma check to fix it.
> 
> --------------------------------------------------------------------------
>   BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
>   PGD 0 P4D 0
>   Oops: 0000 [#1] SMP NOPTI
>   CPU: 9 PID: 510 Comm: ksmd Kdump: loaded Tainted: G OE 4.19.36.bsk.9-amd64 #4.19.36.bsk.9
>   Hardware name: FOXCONN R-5111/GROOT, BIOS IC1B111F 08/17/2019
>   RIP: 0010:try_to_merge_one_page+0xc7/0x760
>   Code: 24 58 65 48 33 34 25 28 00 00 00 89 e8 0f 85 a3 06 00 00 48 83 c4
>         60 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 8b 46 08 a8 01 75 b8 <49>
>         8b 44 24 40 4c 8d 7c 24 20 b9 07 00 00 00 4c 89 e6 4c 89 ff 48
>   RSP: 0018:ffffadbdd9fffdb0 EFLAGS: 00010246
>   RAX: ffffda83ffd4be08 RBX: ffffda83ffd4be40 RCX: 0000002c6e800000
>   RDX: 0000000000000000 RSI: ffffda83ffd4be40 RDI: 0000000000000000
>   RBP: ffffa11939f02ec0 R08: 0000000094e1a447 R09: 00000000abe76577
>   R10: 0000000000000962 R11: 0000000000004e6a R12: 0000000000000000
>   R13: ffffda83b1e06380 R14: ffffa18f31f072c0 R15: ffffda83ffd4be40
>   FS: 0000000000000000(0000) GS:ffffa0da43b80000(0000) knlGS:0000000000000000
>   CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000040 CR3: 0000002c77c0a003 CR4: 00000000007626e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   PKRU: 55555554
>   Call Trace:
>     ? follow_page_pte+0x36d/0x5e0
>     ksm_scan_thread+0x115e/0x1960
>     ? remove_wait_queue+0x60/0x60
>     kthread+0xf5/0x130
>     ? try_to_merge_with_ksm_page+0x90/0x90
>     ? kthread_create_worker_on_cpu+0x70/0x70
>     ret_from_fork+0x1f/0x30
> --------------------------------------------------------------------------
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Signed-off-by: Xiongchun duan <duanxiongchun@xxxxxxxxxxxxx>

^ why this signed-off ?

> ---
>  mm/ksm.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index a558da9e71770..69b2f85e22d5b 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2112,8 +2112,11 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
>  
>  		down_read(&mm->mmap_sem);
>  		vma = find_mergeable_vma(mm, rmap_item->address);
> -		err = try_to_merge_one_page(vma, page,
> -					    ZERO_PAGE(rmap_item->address));
> +		if (vma)
> +			err = try_to_merge_one_page(vma, page,
> +					ZERO_PAGE(rmap_item->address));
> +		else
> +			err = -EFAULT;
>  		up_read(&mm->mmap_sem);
>  		/*
>  		 * In case of failure, the page was not really empty, so we
> 

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb