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@xxxxxxx> > > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > 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
