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? 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
