Re: [PATCH for 4.9-stable] proc/vmcore: fix clearing user buffer by properly using clear_user()

[David Hildenbrand <david@redhat.com>]

On 24.11.21 21:35, David Hildenbrand wrote:
> commit c1e63117711977cc4295b2ce73de29dd17066c82 upstream.
> 
> To clear a user buffer we cannot simply use memset, we have to use
> clear_user().  With a virtio-mem device that registers a vmcore_cb and
> has some logically unplugged memory inside an added Linux memory block,
> I can easily trigger a BUG by copying the vmcore via "cp":
> 
>   systemd[1]: Starting Kdump Vmcore Save Service...
>   kdump[420]: Kdump is using the default log level(3).
>   kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
>   kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
>   kdump[465]: saving vmcore-dmesg.txt complete
>   kdump[467]: saving vmcore
>   BUG: unable to handle page fault for address: 00007f2374e01000
>   #PF: supervisor write access in kernel mode
>   #PF: error_code(0x0003) - permissions violation
>   PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867
>   Oops: 0003 [#1] PREEMPT SMP NOPTI
>   CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014
>   RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86
>   Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7 38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 <49> c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81
>   RSP: 0018:ffffc9000073be08 EFLAGS: 00010212
>   RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000
>   RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008
>   RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50
>   R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000
>   R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8
>   FS:  00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0
>   Call Trace:
>    read_vmcore+0x236/0x2c0
>    proc_reg_read+0x55/0xa0
>    vfs_read+0x95/0x190
>    ksys_read+0x4f/0xc0
>    do_syscall_64+0x3b/0x90
>    entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access
> Prevention (SMAP)", which is used to detect wrong access from the kernel
> to user buffers like this: SMAP triggers a permissions violation on
> wrong access.  In the x86-64 variant of clear_user(), SMAP is properly
> handled via clac()+stac().
> 
> To fix, properly use clear_user() when we're dealing with a user buffer.
> 
> Link: https://lkml.kernel.org/r/20211112092750.6921-1-david@redhat.com
> Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Acked-by: Baoquan He <bhe@redhat.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Philipp Rudo <prudo@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  fs/proc/vmcore.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)

Please ignore this patch, I just resend with the correct --in-reply-to.


-- 
Thanks,

David / dhildenb