Re: [PATCH v1 1/2] mm/hugetlb: fix PTE marker handling in hugetlb_change_protection()

[Mike Kravetz]

On 12/22/22 21:55, David Hildenbrand wrote:
> There are two problematic cases when stumbling over a PTE marker in
> hugetlb_change_protection():
> 
> (1) We protect an uffd-wp PTE marker a second time using uffd-wp: we will
>     end up in the "!huge_pte_none(pte)" case and mess up the PTE marker.
> 
> (2) We unprotect a uffd-wp PTE marker: we will similarly end up in the
>     "!huge_pte_none(pte)" case even though we cleared the PTE, because
>     the "pte" variable is stale. We'll mess up the PTE marker.
> 
> For example, if we later stumble over such a "wrongly modified" PTE marker,
> we'll treat it like a present PTE that maps some garbage page.
> 
> This can, for example, be triggered by mapping a memfd backed by huge
> pages, registering uffd-wp, uffd-wp'ing an unmapped page and (a)
> uffd-wp'ing it a second time; or (b) uffd-unprotecting it; or (c)
> unregistering uffd-wp. Then, ff we trigger fallocate(FALLOC_FL_PUNCH_HOLE)
> on that file range, we will run into a VM_BUG_ON:
> 
> [  195.039560] page:00000000ba1f2987 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x0
> [  195.039565] flags: 0x7ffffc0001000(reserved|node=0|zone=0|lastcpupid=0x1fffff)
> [  195.039568] raw: 0007ffffc0001000 ffffe742c0000008 ffffe742c0000008 0000000000000000
> [  195.039569] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
> [  195.039569] page dumped because: VM_BUG_ON_PAGE(compound && !PageHead(page))
> [  195.039573] ------------[ cut here ]------------
> [  195.039574] kernel BUG at mm/rmap.c:1346!
> [  195.039579] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> [  195.039581] CPU: 7 PID: 4777 Comm: qemu-system-x86 Not tainted 6.0.12-200.fc36.x86_64 #1
> [  195.039583] Hardware name: LENOVO 20WNS1F81N/20WNS1F81N, BIOS N35ET50W (1.50 ) 09/15/2022
> [  195.039584] RIP: 0010:page_remove_rmap+0x45b/0x550
> [  195.039588] Code: [...]
> [  195.039589] RSP: 0018:ffffbc03c3633ba8 EFLAGS: 00010292
> [  195.039591] RAX: 0000000000000040 RBX: ffffe742c0000000 RCX: 0000000000000000
> [  195.039592] RDX: 0000000000000002 RSI: ffffffff8e7aac1a RDI: 00000000ffffffff
> [  195.039592] RBP: 0000000000000001 R08: 0000000000000000 R09: ffffbc03c3633a08
> [  195.039593] R10: 0000000000000003 R11: ffffffff8f146328 R12: ffff9b04c42754b0
> [  195.039594] R13: ffffffff8fcc6328 R14: ffffbc03c3633c80 R15: ffff9b0484ab9100
> [  195.039595] FS:  00007fc7aaf68640(0000) GS:ffff9b0bbf7c0000(0000) knlGS:0000000000000000
> [  195.039596] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  195.039597] CR2: 000055d402c49110 CR3: 0000000159392003 CR4: 0000000000772ee0
> [  195.039598] PKRU: 55555554
> [  195.039599] Call Trace:
> [  195.039600]  <TASK>
> [  195.039602]  __unmap_hugepage_range+0x33b/0x7d0
> [  195.039605]  unmap_hugepage_range+0x55/0x70
> [  195.039608]  hugetlb_vmdelete_list+0x77/0xa0
> [  195.039611]  hugetlbfs_fallocate+0x410/0x550
> [  195.039612]  ? _raw_spin_unlock_irqrestore+0x23/0x40
> [  195.039616]  vfs_fallocate+0x12e/0x360
> [  195.039618]  __x64_sys_fallocate+0x40/0x70
> [  195.039620]  do_syscall_64+0x58/0x80
> [  195.039623]  ? syscall_exit_to_user_mode+0x17/0x40
> [  195.039624]  ? do_syscall_64+0x67/0x80
> [  195.039626]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> [  195.039628] RIP: 0033:0x7fc7b590651f
> [  195.039653] Code: [...]
> [  195.039654] RSP: 002b:00007fc7aaf66e70 EFLAGS: 00000293 ORIG_RAX: 000000000000011d
> [  195.039655] RAX: ffffffffffffffda RBX: 0000558ef4b7f370 RCX: 00007fc7b590651f
> [  195.039656] RDX: 0000000018000000 RSI: 0000000000000003 RDI: 000000000000000c
> [  195.039657] RBP: 0000000008000000 R08: 0000000000000000 R09: 0000000000000073
> [  195.039658] R10: 0000000008000000 R11: 0000000000000293 R12: 0000000018000000
> [  195.039658] R13: 00007fb8bbe00000 R14: 000000000000000c R15: 0000000000001000
> [  195.039661]  </TASK>
> 
> Fix it by not going into the "!huge_pte_none(pte)" case if we stumble
> over an exclusive marker. spin_unlock() + continue would get the job
> done.
> 
> However, instead, make it clearer that there are no fall-through
> statements: we process each case (hwpoison, migration, marker, !none, none)
> and then unlock the page table to continue with the next PTE. Let's
> avoid "continue" statements and use a single spin_unlock() at the end.
> 
> Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)

Thanks for the fix, and restructuring to make the code more clear.

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz