linux-kernel - Re: BUG: unable to handle kernel paging request in fuse_copy

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Date: Fri, 22 Mar 2024 22:18:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: xingwei lee <xrivendell7@...il.com>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, samsun1006219@...il.com,
 syzkaller-bugs@...glegroups.com, linux-mm <linux-mm@...ck.org>,
 Mike Rapoport <rppt@...nel.org>
Subject: Re: BUG: unable to handle kernel paging request in fuse_copy_do

On 22.03.24 22:13, Miklos Szeredi wrote:
> On Fri, 22 Mar 2024 at 22:08, David Hildenbrand <david@...hat.com> wrote:
>>
>> On 22.03.24 20:46, Miklos Szeredi wrote:
>>> On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@...hat.com> wrote:
>>>
>>>> But at least the vmsplice() just seems to work. Which is weird, because
>>>> GUP-fast should not apply (page not faulted in?)
>>>
>>> But it is faulted in, and that indeed seems to be the root cause.
>>
>> secretmem mmap() won't populate the page tables. So it's not faulted in yet.
>>
>> When we GUP via vmsplice, GUP-fast should not find it in the page tables
>> and fallback to slow GUP.
>>
>> There, we seem to pass check_vma_flags(), trigger faultin_page() to
>> fault it in, and then find it via follow_page_mask().
>>
>> ... and I wonder how we manage to skip check_vma_flags(), or otherwise
>> managed to GUP it.
>>
>> vmsplice() should, in theory, never succeed here.
>>
>> Weird :/
>>
>>> Improved repro:
>>>
>>> #define _GNU_SOURCE
>>>
>>> #include <fcntl.h>
>>> #include <unistd.h>
>>> #include <stdio.h>
>>> #include <errno.h>
>>> #include <sys/mman.h>
>>> #include <sys/syscall.h>
>>>
>>> int main(void)
>>> {
>>>           int fd1, fd2;
>>>           int pip[2];
>>>           struct iovec iov;
>>>           char *addr;
>>>           int ret;
>>>
>>>           fd1 = syscall(__NR_memfd_secret, 0);
>>>           addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd1, 0);
>>>           ftruncate(fd1, 7);
>>>           addr[0] = 1; /* fault in page */
> 
> Here the page is faulted in and GUP-fast will find it.  It's not in
> the kernel page table, but it is in the user page table, which is what
> matter for GUP.

Trust me, I know the GUP code very well :P

gup_pte_range -- GUP fast -- contains:

if (unlikely(folio_is_secretmem(folio))) {
	gup_put_folio(folio, 1, flags);
	goto pte_unmap;
}

So we "should" be rejecting any secretmem folios and fallback to GUP slow.


.. we don't check the same in gup_huge_pmd(), but we shouldn't ever see 
THP in secretmem code.

-- 
Cheers,

David / dhildenb