drm/msm: Fix mmap to include VM_IO and VM_DONTDUMP

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.