Subject: CVE-2021-47531: drm/msm: Fix mmap to include VM_IO and VM_DONTDUMP

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

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

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

drm/msm: Fix mmap to include VM_IO and VM_DONTDUMP

In commit 510410bfc034 ("drm/msm: Implement mmap as GEM object
function") we switched to a new/cleaner method of doing things. That's
good, but we missed a little bit.

Before that commit, we used to _first_ run through the
drm_gem_mmap_obj() case where `obj->funcs->mmap()` was NULL. That meant
that we ran:

  vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
  vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
  vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

...and _then_ we modified those mappings with our own. Now that
`obj->funcs->mmap()` is no longer NULL we don't run the default
code. It looks like the fact that the vm_flags got VM_IO / VM_DONTDUMP
was important because we're now getting crashes on Chromebooks that
use ARC++ while logging out. Specifically a crash that looks like this
(this is on a 5.10 kernel w/ relevant backports but also seen on a
5.15 kernel):

  Unable to handle kernel paging request at virtual address ffffffc008000000
  Mem abort info:
    ESR = 0x96000006
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
  Data abort info:
    ISV = 0, ISS = 0x00000006
    CM = 0, WnR = 0
  swapper pgtable: 4k pages, 39-bit VAs, pgdp=000000008293d000
  [ffffffc008000000] pgd=00000001002b3003, p4d=00000001002b3003,
                     pud=00000001002b3003, pmd=0000000000000000
  Internal error: Oops: 96000006 [#1] PREEMPT SMP
  [...]
  CPU: 7 PID: 15734 Comm: crash_dump64 Tainted: G W 5.10.67 #1 [...]
  Hardware name: Qualcomm Technologies, Inc. sc7280 IDP SKU2 platform (DT)
  pstate: 80400009 (Nzcv daif +PAN -UAO -TCO BTYPE=--)
  pc : __arch_copy_to_user+0xc0/0x30c
  lr : copyout+0xac/0x14c
  [...]
  Call trace:
   __arch_copy_to_user+0xc0/0x30c
   copy_page_to_iter+0x1a0/0x294
   process_vm_rw_core+0x240/0x408
   process_vm_rw+0x110/0x16c
   __arm64_sys_process_vm_readv+0x30/0x3c
   el0_svc_common+0xf8/0x250
   do_el0_svc+0x30/0x80
   el0_svc+0x10/0x1c
   el0_sync_handler+0x78/0x108
   el0_sync+0x184/0x1c0
  Code: f8408423 f80008c3 910020c6 36100082 (b8404423)

Let's add the two flags back in.

While we're at it, the fact that we aren't running the default means
that we _don't_ need to clear out VM_PFNMAP, so remove that and save
an instruction.

NOTE: it was confirmed that VM_IO was the important flag to fix the
problem I was seeing, but adding back VM_DONTDUMP seems like a sane
thing to do so I'm doing that too.

The Linux kernel CVE team has assigned CVE-2021-47531 to this issue.

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

	Issue introduced in 5.15 with commit 510410bfc034 and fixed in 5.15.7 with commit 8e2b7fe5e8a4
	Issue introduced in 5.15 with commit 510410bfc034 and fixed in 5.16 with commit 3466d9e217b3

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-2021-47531
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:
	drivers/gpu/drm/msm/msm_gem.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/8e2b7fe5e8a4be5e571561d9afcfbd92097288ba
	https://git.kernel.org/stable/c/3466d9e217b337bf473ee629c608e53f9f3ab786