Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag

On 13.04.22 15:49, Catalin Marinas wrote:

The aim of such policy is to prevent a user task from inadvertently
creating an executable mapping that is or was writeable (and
subsequently made read-only).

An example of mmap() returning -EACCESS if the policy is enabled:

	mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, flags, 0, 0);

Similarly, mprotect() would return -EACCESS below:

	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
	mprotect(addr, size, PROT_READ | PROT_WRITE | PROT_EXEC);

With the past vma writeable permission tracking, mprotect() below would
also fail with -EACCESS:

	addr = mmap(0, size, PROT_READ | PROT_WRITE, flags, 0, 0);
	mprotect(addr, size, PROT_READ | PROT_EXEC);

While the above could be achieved by checking PROT_WRITE & PROT_EXEC on
mmap/mprotect and denying mprotect(PROT_EXEC) altogether (current
systemd MDWE approach via SECCOMP BPF filters), we want the following
scenario to succeed:

	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
	mprotect(addr, size, PROT_READ | PROT_EXEC | PROT_BTI);

where PROT_BTI enables branch tracking identification on arm64.

The choice for a DENY_WRITE_EXEC personality flag, inherited on fork()
and execve(), was made by analogy to READ_IMPLIES_EXEC.

Note that it is sufficient to check for VM_WAS_WRITE in
map_deny_write_exec() as this flag is always set on VM_WRITE mappings.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

How does this interact with get_user_pages(FOLL_WRITE|FOLL_FORCE) on a
VMA that is VM_MAYWRITE but not VM_WRITE? Is it handled accordingly?

Note that in the (FOLL_WRITE|FOLL_FORCE) we only require VM_MAYWRITE on
the vma and trigger a write fault. As the VMA is not VM_WRITE, we won't
actually map the PTE writable, but set it dirty. GUP will retry, find a
R/O pte that is dirty and where it knows that it broke COW and will
allow the read access, although the PTE is R/O.

That mechanism is required to e.g., set breakpoints in R/O MAP_PRIVATE
kernel sections, but it's used elsewhere for page pinning as well.

My gut feeling is that GUP(FOLL_WRITE|FOLL_FORCE) could be used right
now to bypass that mechanism, I might be wrong.

-- 
Thanks,

David / dhildenb


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