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Subject: Re: [PATCH] mm: madvise: fix vma user-after-free
To:     Yang Shi <shy828301@gmail.com>, jack@suse.cz,
        akpm@linux-foundation.org
Cc:     stable@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200816141204.162624-1-shy828301@gmail.com>
From:   David Hildenbrand <david@redhat.com>
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Date:   Tue, 8 Sep 2020 12:48:48 +0200
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On 16.08.20 16:12, Yang Shi wrote:
> The syzbot reported the below use-after-free:
> 
> BUG: KASAN: use-after-free in madvise_willneed mm/madvise.c:293 [inline]
> BUG: KASAN: use-after-free in madvise_vma mm/madvise.c:942 [inline]
> BUG: KASAN: use-after-free in do_madvise.part.0+0x1c8b/0x1cf0 mm/madvise.c:1145
> Read of size 8 at addr ffff8880a6163eb0 by task syz-executor.0/9996
> 
> CPU: 0 PID: 9996 Comm: syz-executor.0 Not tainted 5.9.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x18f/0x20d lib/dump_stack.c:118
>  print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
>  __kasan_report mm/kasan/report.c:513 [inline]
>  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
>  madvise_willneed mm/madvise.c:293 [inline]
>  madvise_vma mm/madvise.c:942 [inline]
>  do_madvise.part.0+0x1c8b/0x1cf0 mm/madvise.c:1145
>  do_madvise mm/madvise.c:1169 [inline]
>  __do_sys_madvise mm/madvise.c:1171 [inline]
>  __se_sys_madvise mm/madvise.c:1169 [inline]
>  __x64_sys_madvise+0xd9/0x110 mm/madvise.c:1169
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x45d4d9
> Code: 5d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 2b b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f04f7464c78 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 0000000000020800 RCX: 000000000045d4d9
> RDX: 0000000000000003 RSI: 0000000000600003 RDI: 0000000020000000
> RBP: 000000000118d020 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cfec
> R13: 00007ffc579cce7f R14: 00007f04f74659c0 R15: 000000000118cfec
> 
> Allocated by task 9992:
>  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
>  kasan_set_track mm/kasan/common.c:56 [inline]
>  __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
>  slab_post_alloc_hook mm/slab.h:518 [inline]
>  slab_alloc mm/slab.c:3312 [inline]
>  kmem_cache_alloc+0x138/0x3a0 mm/slab.c:3482
>  vm_area_alloc+0x1c/0x110 kernel/fork.c:347
>  mmap_region+0x8e5/0x1780 mm/mmap.c:1743
>  do_mmap+0xcf9/0x11d0 mm/mmap.c:1545
>  vm_mmap_pgoff+0x195/0x200 mm/util.c:506
>  ksys_mmap_pgoff+0x43a/0x560 mm/mmap.c:1596
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Freed by task 9992:
>  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
>  kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
>  kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
>  __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:422
>  __cache_free mm/slab.c:3418 [inline]
>  kmem_cache_free.part.0+0x67/0x1f0 mm/slab.c:3693
>  remove_vma+0x132/0x170 mm/mmap.c:184
>  remove_vma_list mm/mmap.c:2613 [inline]
>  __do_munmap+0x743/0x1170 mm/mmap.c:2869
>  do_munmap mm/mmap.c:2877 [inline]
>  mmap_region+0x257/0x1780 mm/mmap.c:1716
>  do_mmap+0xcf9/0x11d0 mm/mmap.c:1545
>  vm_mmap_pgoff+0x195/0x200 mm/util.c:506
>  ksys_mmap_pgoff+0x43a/0x560 mm/mmap.c:1596
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> It is because vma is accessed after releasing mmap_sem, but someone else
> acquired the mmap_sem and the vma is gone.
> 
> Releasing mmap_sem after accessing vma should fix the problem.
> 
> Fixes: 692fe62433d4c ("mm: Handle MADV_WILLNEED through vfs_fadvise()")
> Reported-by: syzbot+b90df26038d1d5d85c97@syzkaller.appspotmail.com
> Cc: Jan Kara <jack@suse.cz>
> Cc: <stable@vger.kernel.org> v5.4+
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  mm/madvise.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index dd1d43cf026d..d4aa5f776543 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -289,9 +289,9 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  	 */
>  	*prev = NULL;	/* tell sys_madvise we drop mmap_lock */
>  	get_file(file);
> -	mmap_read_unlock(current->mm);
>  	offset = (loff_t)(start - vma->vm_start)
>  			+ ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> +	mmap_read_unlock(current->mm);
>  	vfs_fadvise(file, offset, end - start, POSIX_FADV_WILLNEED);
>  	fput(file);
>  	mmap_read_lock(current->mm);
> 

Late to the party, nice finding and fix

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb