Re: [PATCH 7/7] mm: return an ERR_PTR from __filemap_get_folio

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

On Fri, Mar 10, 2023 at 08:00:23AM +0100, Christoph Hellwig wrote:
> On Fri, Mar 10, 2023 at 01:31:37PM +0900, Naoya Horiguchi wrote:
> > On Sat, Jan 21, 2023 at 07:57:55AM +0100, Christoph Hellwig wrote:
> > > Instead of returning NULL for all errors, distinguish between:
> > > 
> > >  - no entry found and not asked to allocated (-ENOENT)
> > >  - failed to allocate memory (-ENOMEM)
> > >  - would block (-EAGAIN)
> > > 
> > > so that callers don't have to guess the error based on the passed
> > > in flags.
> > > 
> > > Also pass through the error through the direct callers:
> > > filemap_get_folio, filemap_lock_folio filemap_grab_folio
> > > and filemap_get_incore_folio.
> > > 
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > 
> > Hello,
> > 
> > I found a NULL pointer dereference issue related to this patch,
> > so let me share it.
> > 
> > Here is the bug message (I used akpm/mm-unstable on Mar 9):
> > 
> > [ 2871.648659] BUG: kernel NULL pointer dereference, address: 0000000000000000
> > [ 2871.651286] #PF: supervisor read access in kernel mode
> > [ 2871.653231] #PF: error_code(0x0000) - not-present page
> > [ 2871.655170] PGD 80000001517dd067 P4D 80000001517dd067 PUD 1491d1067 PMD 0
> > [ 2871.657739] Oops: 0000 [#1] PREEMPT SMP PTI
> > [ 2871.659329] CPU: 4 PID: 1599 Comm: page-types Tainted: G            E    N 6.3.0-rc1-v6.3-rc1-230309-1629-189-ga71a7+ #36
> > [ 2871.663362] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014
> > [ 2871.666507] RIP: 0010:mincore_page+0x19/0x90
> > [ 2871.668086] Code: cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 41 54 55 53 e8 92 2b 03 00 48 3d 00 f0 ff ff 77 54 48 89 c3 <48> 8b 00 48 c1 e8 02 89 c5 83 e5 01 75 21 8b 43 34 85 c0 74 47 f0
> > [ 2871.678313] RSP: 0018:ffffbe57c203fd00 EFLAGS: 00010207
> > [ 2871.681422] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > [ 2871.685609] RDX: 0000000000000000 RSI: ffff9f59ca1506d8 RDI: ffff9f59ce7c2880
> > [ 2871.689599] RBP: 0000000000000000 R08: 00007f9f14200000 R09: ffff9f59c9078508
> > [ 2871.693295] R10: 00007f9ed4400000 R11: 0000000000000000 R12: 0000000000000200
> > [ 2871.695969] R13: 0000000000000001 R14: ffff9f59c9ef4450 R15: ffff9f59c4ac9000
> > [ 2871.699927] FS:  00007f9ed47ee740(0000) GS:ffff9f5abbc00000(0000) knlGS:0000000000000000
> > [ 2871.703969] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 2871.706689] CR2: 0000000000000000 CR3: 0000000149ffe006 CR4: 0000000000170ee0
> > [ 2871.709923] DR0: ffffffff91531760 DR1: ffffffff91531761 DR2: ffffffff91531762
> > [ 2871.713424] DR3: ffffffff91531763 DR6: 00000000ffff0ff0 DR7: 0000000000000600
> > [ 2871.716758] Call Trace:
> > [ 2871.717998]  <TASK>
> > [ 2871.719008]  __mincore_unmapped_range+0x6e/0xd0
> > [ 2871.721220]  mincore_unmapped_range+0x16/0x30
> > [ 2871.723288]  walk_pgd_range+0x485/0x9e0
> > [ 2871.725128]  __walk_page_range+0x195/0x1b0
> > [ 2871.727224]  walk_page_range+0x151/0x180
> > [ 2871.728883]  __do_sys_mincore+0xec/0x2b0
> > [ 2871.730707]  do_syscall_64+0x3a/0x90
> > [ 2871.732179]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> > [ 2871.734148] RIP: 0033:0x7f9ed443f4ab
> > [ 2871.735548] Code: 73 01 c3 48 8b 0d 75 99 1b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 1b 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 45 99 1b 00 f7 d8 64 89 01 48
> > [ 2871.742194] RSP: 002b:00007ffe924d72b8 EFLAGS: 00000206 ORIG_RAX: 000000000000001b
> > [ 2871.744787] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f9ed443f4ab
> > [ 2871.747186] RDX: 00007ffe92557300 RSI: 0000000000200000 RDI: 00007f9ed4200000
> > [ 2871.749404] RBP: 00007ffe92567330 R08: 0000000000000005 R09: 0000000000000000
> > [ 2871.751683] R10: 00007f9ed4405d68 R11: 0000000000000206 R12: 00007ffe925674b8
> > [ 2871.753925] R13: 0000000000404af1 R14: 000000000040ad78 R15: 00007f9ed4833000
> > [ 2871.756493]  </TASK>
> > 
> > The precedure to reproduce this is (1) punch hole some page in a shmem
> > file, then (2) call mincore() over the punch-holed address range. 
> > 
> > I confirmed that filemap_get_incore_folio() (actually filemap_get_entry()
> > inside it) returns NULL in that case, so we unexpectedly enter the following
> > if-block for the "not found" case.
> 
> Yes.  Please try this fix:
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index c7160070b9daa9..b76a65ac28b319 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -390,6 +390,8 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping,
>  	struct swap_info_struct *si;
>  	struct folio *folio = filemap_get_entry(mapping, index);
>  
> +	if (!folio)
> +		return ERR_PTR(-ENOENT);
>  	if (!xa_is_value(folio))
>  		return folio;
>  	if (!shmem_mapping(mapping))

Confirmed the fix, thank you very much!

- Naoya Horiguchi