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

From: David Hildenbrand
Date: Wed Nov 24 2021 - 15:40:36 EST


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@xxxxxxxxxx
> Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages")
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Philipp Rudo <prudo@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> 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