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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Apr 21, 2022 at 06:37:49PM +0100, David Hildenbrand wrote:
> 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?

For now, that's just about VM_WRITE. Most vmas are VM_MAYWRITE, so we
can't really have MAYWRITE^EXEC. The basic feature aims to avoid user
vulnerabilities where a buffer is mapped both writeable and executable.
Of course, it can be expanded with additional prctl() flags to cover
other cases.

> 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.

GUP can be used to bypass this. But if an attacker can trigger such GUP
paths via a syscall (e.g. ptrace(PTRACE_POKEDATA)), I think we need the
checks on those paths (and reject the syscall) rather than on
mmap/mprotect(). This would be covered by something like CAP_SYS_PTRACE.

Not sure what would break if we prevent GUP(FOLL_WRITE|FOLL_FORCE) when
the vma is !VM_WRITE, basically removing FOLL_FORCE. I guess for
ptrace() and uprobes that's fine. We could also make this only about
VM_EXEC rather than VM_WRITE, though  we'd probably need to set
VM_WAS_WRITE if we ever had a GUP(FOLL_WRITE|FOLL_FORCE) in order to
prevent a subsequent mprotect(PROT_EXEC).

Anyway, this can be a new flag. My first aim is to get the basics
working similarly to systemd's MDWE.

-- 
Catalin

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