Subject: Re: [syzbot] linux-next boot error: BUG: unable to handle kernel paging request in kernel_execve

From: Ira Weiny <[email-masked]>
Date: Thu, 11 Aug 2022 11:51:34 -0700

On Thu, Aug 11, 2022 at 10:39:29AM -0700, Ira wrote:
> On Thu, Aug 11, 2022 at 08:33:16AM -0700, Kees Cook wrote:
> > Hi Fabio,
> > 
> > It seems likely that the kmap change[1] might be causing this crash. Is
> > there a boot-time setup race between kmap being available and early umh
> > usage?
> 
> I don't see how this is a setup problem with the config reported here.
> 
> CONFIG_64BIT=y
> 
> ...and HIGHMEM is not set.
> ...and PREEMPT_RT is not set.
> 
> So the kmap_local_page() call in that stack should be a page_address() only.
> 
> I think the issue must be some sort of race which was being prevented because
> of the preemption and/or pagefault disable built into kmap_atomic().
> 
> Is this reproducable?
> 
> The hunk below will surely fix it but I think the pagefault_disable() is
> the only thing that is required.  It would be nice to test it.

Fabio and I discussed this.  And he also mentioned that pagefault_disable() is
all that is required.

Do we have a way to test this?
Ira

> 
> Ira
> 
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index b51dd14e7388..3da588c858ca 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -640,7 +640,11 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
>                 if (!page)
>                         return -E2BIG;
>                 flush_arg_page(bprm, pos & PAGE_MASK, page);
> +               preempt_disable();
> +               pagefault_disable();
>                 memcpy_to_page(page, offset_in_page(pos), arg, bytes_to_copy);
> +               pagefault_enable();
> +               preempt_enable();
>                 put_arg_page(page);
>         }
>  
> 
> > 
> > -Kees
> > 
> > [1] https://git.kernel.org/linus/c6e8e36c6ae4b11bed5643317afb66b6c3cadba8
> > 
> > On Thu, Aug 11, 2022 at 12:29:34AM -0700, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    bc6c6584ffb2 Add linux-next specific files for 20220810
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=115034c3080000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=5784be4315a4403b
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=3250d9c8925ef29e975f
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: [email-masked]
> > > 
> > > BUG: unable to handle page fault for address: ffffdc0000000000
> > > #PF: supervisor read access in kernel mode
> > > #PF: error_code(0x0000) - not-present page
> > > PGD 11826067 P4D 11826067 PUD 0 
> > > Oops: 0000 [#1] PREEMPT SMP KASAN
> > > CPU: 0 PID: 1100 Comm: kworker/u4:5 Not tainted 5.19.0-next-20220810-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022
> > > RIP: 0010:strnlen+0x3b/0x70 lib/string.c:504
> > > Code: 74 3c 48 bb 00 00 00 00 00 fc ff df 49 89 fc 48 89 f8 eb 09 48 83 c0 01 48 39 e8 74 1e 48 89 c2 48 89 c1 48 c1 ea 03 83 e1 07 <0f> b6 14 1a 38 ca 7f 04 84 d2 75 11 80 38 00 75 d9 4c 29 e0 48 83
> > > RSP: 0000:ffffc90005c5fe10 EFLAGS: 00010246
> > > RAX: ffff000000000000 RBX: dffffc0000000000 RCX: 0000000000000000
> > > RDX: 1fffe00000000000 RSI: 0000000000020000 RDI: ffff000000000000
> > > RBP: ffff000000020000 R08: 0000000000000005 R09: 0000000000000000
> > > R10: 0000000000000006 R11: 0000000000000000 R12: ffff000000000000
> > > R13: ffff88814764cc00 R14: ffff000000000000 R15: ffff88814764cc00
> > > FS:  0000000000000000(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: ffffdc0000000000 CR3: 000000000bc8e000 CR4: 00000000003506f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > Call Trace:
> > >  <TASK>
> > >  strnlen include/linux/fortify-string.h:119 [inline]
> > >  copy_string_kernel+0x26/0x250 fs/exec.c:616
> > >  copy_strings_kernel+0xb3/0x190 fs/exec.c:655
> > >  kernel_execve+0x377/0x500 fs/exec.c:1998
> > >  call_usermodehelper_exec_async+0x2e3/0x580 kernel/umh.c:112
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
> > >  </TASK>
> > > Modules linked in:
> > > CR2: ffffdc0000000000
> > > ---[ end trace 0000000000000000 ]---
> > > RIP: 0010:strnlen+0x3b/0x70 lib/string.c:504
> > > Code: 74 3c 48 bb 00 00 00 00 00 fc ff df 49 89 fc 48 89 f8 eb 09 48 83 c0 01 48 39 e8 74 1e 48 89 c2 48 89 c1 48 c1 ea 03 83 e1 07 <0f> b6 14 1a 38 ca 7f 04 84 d2 75 11 80 38 00 75 d9 4c 29 e0 48 83
> > > RSP: 0000:ffffc90005c5fe10 EFLAGS: 00010246
> > > RAX: ffff000000000000 RBX: dffffc0000000000 RCX: 0000000000000000
> > > RDX: 1fffe00000000000 RSI: 0000000000020000 RDI: ffff000000000000
> > > RBP: ffff000000020000 R08: 0000000000000005 R09: 0000000000000000
> > > R10: 0000000000000006 R11: 0000000000000000 R12: ffff000000000000
> > > R13: ffff88814764cc00 R14: ffff000000000000 R15: ffff88814764cc00
> > > FS:  0000000000000000(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: ffffdc0000000000 CR3: 000000000bc8e000 CR4: 00000000003506f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > ----------------
> > > Code disassembly (best guess):
> > >    0:	74 3c                	je     0x3e
> > >    2:	48 bb 00 00 00 00 00 	movabs $0xdffffc0000000000,%rbx
> > >    9:	fc ff df
> > >    c:	49 89 fc             	mov    %rdi,%r12
> > >    f:	48 89 f8             	mov    %rdi,%rax
> > >   12:	eb 09                	jmp    0x1d
> > >   14:	48 83 c0 01          	add    $0x1,%rax
> > >   18:	48 39 e8             	cmp    %rbp,%rax
> > >   1b:	74 1e                	je     0x3b
> > >   1d:	48 89 c2             	mov    %rax,%rdx
> > >   20:	48 89 c1             	mov    %rax,%rcx
> > >   23:	48 c1 ea 03          	shr    $0x3,%rdx
> > >   27:	83 e1 07             	and    $0x7,%ecx
> > > * 2a:	0f b6 14 1a          	movzbl (%rdx,%rbx,1),%edx <-- trapping instruction
> > >   2e:	38 ca                	cmp    %cl,%dl
> > >   30:	7f 04                	jg     0x36
> > >   32:	84 d2                	test   %dl,%dl
> > >   34:	75 11                	jne    0x47
> > >   36:	80 38 00             	cmpb   $0x0,(%rax)
> > >   39:	75 d9                	jne    0x14
> > >   3b:	4c 29 e0             	sub    %r12,%rax
> > >   3e:	48                   	rex.W
> > >   3f:	83                   	.byte 0x83
> > > 
> > > 
> > > ---
> > > This report is generated by a bot. It may contain errors.
> > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > syzbot engineers can be reached at [email-masked].
> > > 
> > > syzbot will keep track of this issue. See:
> > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > 
> > -- 
> > Kees Cook