Messages in this thread | | | Date | Tue, 15 Nov 2022 21:31:02 +0200 | Subject | Re: [PATCH v1 1/2] mm: Implement memory-deny-write-execute as a prctl | From | Topi Miettinen <> |
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On 15.11.2022 17.35, Catalin Marinas wrote: > On Sat, Nov 12, 2022 at 08:11:24AM +0200, Topi Miettinen wrote: >> On 10.11.2022 14.03, Catalin Marinas wrote: >>> On Thu, Nov 10, 2022 at 11:27:14AM +0000, Joey Gouly wrote: >>>> On Fri, Oct 28, 2022 at 11:51:00AM -0700, Kees Cook wrote: >>>>> On Wed, Oct 26, 2022 at 04:04:56PM +0100, Joey Gouly wrote: >>>>>> diff --git a/mm/mmap.c b/mm/mmap.c >>>>>> index 099468aee4d8..42eaf6683216 100644 >>>>>> --- a/mm/mmap.c >>>>>> +++ b/mm/mmap.c >>>>>> @@ -1409,6 +1409,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr, >>>>>> vm_flags |= VM_NORESERVE; >>>>>> } >>>>>> + if (map_deny_write_exec(NULL, vm_flags)) >>>>>> + return -EACCES; >>>>>> + >>>>> >>>>> This seems like the wrong place to do the check -- that the vma argument >>>>> is a hard-coded "NULL" is evidence that something is wrong. Shouldn't >>>>> it live in mmap_region()? What happens with MAP_FIXED, when there is >>>>> an underlying vma? i.e. an MAP_FIXED will, I think, bypass the intended >>>>> check. For example, we had "c" above: >>>>> >>>>> c) mmap(PROT_READ); >>>>> mprotect(PROT_READ|PROT_EXEC); // fails >>>>> >>>>> But this would allow another case: >>>>> >>>>> e) addr = mmap(..., PROT_READ, ...); >>>>> mmap(addr, ..., PROT_READ | PROT_EXEC, MAP_FIXED, ...); // passes >>>> >>>> I can move the check into mmap_region() but it won't fix the MAP_FIXED >>>> example that you showed here. >>>> >>>> mmap_region() calls do_mas_munmap(..) which will unmap overlapping regions. >>>> However the `vma` for the 'old' region is not kept around, and a new vma will >>>> be allocated later on "vma = vm_area_alloc(mm);", and the vm_flags are just set >>>> to what is passed into mmap_region(), so map_deny_write_exec(vma, vm_flags) >>>> will just be as good as passing NULL. >>>> >>>> It's possible to save the vm_flags from the region that is unmapped, but Catalin >>>> suggested it might be better if that is part of a later extension, what do you >>>> think? >>> >>> I thought initially we should keep the behaviour close to what systemd >>> achieves via SECCOMP while only relaxing an mprotect(PROT_EXEC) if the >>> vma is already executable (i.e. check actual permission change not just >>> the PROT_* flags). >>> >>> We could pass the old vm_flags for that region (and maybe drop the vma >>> pointer entirely, just check old and new vm_flags). But this feels like >>> tightening slightly systemd's MDWE approach. If user-space doesn't get >>> confused by this, I'm fine to go with it. Otherwise we can add a new >>> flag later for this behaviour >>> >>> I guess that's more of a question for Topi on whether point tightening >>> point (e) is feasible/desirable. >> >> I think we want 1:1 compatibility with seccomp() for the basic version, so >> MAP_FIXED shouldn't change the verdict. Later we can introduce more versions >> (perhaps even less strict, too) when it's requested by configuration, like >> MemoryDenyWriteExecute=[relaxed | strict]. > > Are you ok with allowing mprotect(PROT_EXEC|PROT_BTI) if the mapping is > already PROT_EXEC? Or you'd rather reject that as well? >
I think that it's OK to allow that. It's an incompatible change, but it shouldn't break anything.
-Topi
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