CVE-2021-47531

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. Thats 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 dont run the default code. It looks like the fact that the vm_flags got VM_IO / VM_DONTDUMP was important because were 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)

Lets add the two flags back in.

While were at it, the fact that we arent running the default means that we dont 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 Im doing that too.

Affected Software

References

• https://git.kernel.org/stable/c/3466d9e217b337bf473ee629c608e53f9f3ab786
• https://git.kernel.org/stable/c/8e2b7fe5e8a4be5e571561d9afcfbd92097288ba