On 22.04.22 12:28, Catalin Marinas wrote: > 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. > > I was told that RDMA uses FOLL_FORCE|FOLL_WRITE and is available to unprivileged users. -- Thanks, David / dhildenb
