Subject: CVE-2021-47566: proc/vmcore: fix clearing user buffer by properly using clear_user()

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

proc/vmcore: fix clearing user buffer by properly using clear_user()

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.

The Linux kernel CVE team has assigned CVE-2021-47566 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 3.0 with commit 997c136f518c and fixed in 4.4.294 with commit a9e164bd160b
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 4.9.292 with commit 33a7d698f30f
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 4.14.257 with commit 99d348b82bcb
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 4.19.219 with commit 9ef384ed300d
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 5.4.163 with commit fd7974c547ab
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 5.10.83 with commit a8a917058faf
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 5.15.6 with commit 7b3a34f08d11
	Issue introduced in 3.0 with commit 997c136f518c and fixed in 5.16 with commit c1e631177119

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47566
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/proc/vmcore.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a9e164bd160be8cbee1df70acb379129e3cd2e7c
	https://git.kernel.org/stable/c/33a7d698f30fa0b99d50569e9909d3baa65d8f6a
	https://git.kernel.org/stable/c/99d348b82bcb36171f24411d3f1a15706a2a937a
	https://git.kernel.org/stable/c/9ef384ed300d1bcfb23d0ab0b487d544444d4b52
	https://git.kernel.org/stable/c/fd7974c547abfb03072a4ee706d3a6f182266f89
	https://git.kernel.org/stable/c/a8a917058faf4abaec9fb614bb6d5f8fe3529ec6
	https://git.kernel.org/stable/c/7b3a34f08d11e7f05cd00b8e09adaa15192f0ad1
	https://git.kernel.org/stable/c/c1e63117711977cc4295b2ce73de29dd17066c82