On Thu, Mar 30, 2023 at 01:47:48PM -0700, Andy Lutomirski wrote: > > > > On Mar 30, 2023, at 12:25 PM, Lorenzo Stoakes <lstoakes@xxxxxxxxx> wrote: > > > > On Sat, Mar 25, 2023 at 02:51:05PM +0000, Lorenzo Stoakes wrote: > >>> On Fri, Mar 24, 2023 at 01:36:46PM -0700, Andrew Morton wrote: > >>> (switched to email. Please respond via emailed reply-to-all, not via the > >>> bugzilla web interface). > >>> > >>>> On Fri, 24 Mar 2023 03:34:23 +0000 bugzilla-daemon@xxxxxxxxxx wrote: > >>> > >>>> https://bugzilla.kernel.org/show_bug.cgi?id=217238 > >>>> > >>>> Bug ID: 217238 > >>>> Summary: Creating shared read-only map is denied after add > >>>> write seal to a memfd > >>>> Product: Memory Management > >>>> Version: 2.5 > >>>> Kernel Version: 6.2.8 > >>>> Hardware: All > >>>> OS: Linux > >>>> Tree: Mainline > >>>> Status: NEW > >>>> Severity: normal > >>>> Priority: P1 > >>>> Component: Other > >>>> Assignee: akpm@xxxxxxxxxxxxxxxxxxxx > >>>> Reporter: yshuiv7@xxxxxxxxx > >>>> Regression: No > >>>> > >>>> Test case: > >>>> > >>>> int main() { > >>>> int fd = memfd_create("test", MFD_ALLOW_SEALING); > >>>> write(fd, "test", 4); > >>>> fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE); > >>>> > >>>> void *ret = mmap(NULL, 4, PROT_READ, MAP_SHARED, fd, 0); > >>>> } > >>>> > >>>> This fails with EPERM. This is in contradiction with what's described in the > >>>> documentation of F_SEAL_WRITE. > >>>> > >>>> -- > >>>> You may reply to this email to add a comment. > >>>> > >>>> You are receiving this mail because: > >>>> You are the assignee for the bug. > >>> > >> > >> This issue seems to be the result of the use of the memfd's shmem region's > >> page cache object (struct address_space)'s i_mmap_writable field to denote > >> whether it is write-sealed. > >> > >> The kernel assumes that a VM_SHARED mapping might become writable at any > >> time via mprotect(), therefore treats VM_SHARED mappings as if they were > >> writable as far as i_mmap_writable is concerned (this field's primary use > >> is to determine whether, for architectures that require it, flushing must > >> occur if this is set to avoid aliasing, see filemap_read() for example). > >> > >> In theory we could convert all such checks to VM_SHARED | VM_WRITE > >> (importantly including on fork) and then update mprotect() to check > >> mapping_map_writable() if a user tries to make unwritable memory > >> writable. > >> > > Unless I’m missing something, we have VM_MAYWRITE for almost exactly this purpose. Can’t we just make a shared mapping with both of these bits clear? > That's a good point, and there's definitely quite a few places where VM_SHARED is simply taken to imply writable which is a little irksome, however sprinkling some VM_MAYWRITE checks in these places would resolve that. Let me take a look into this and perhaps spin up a RFC to iron out the details if this is indeed viable. > >> I suspect however there are reasons relating to locking that make it > >> unreasonable to try to do this, but I may be mistaken (others might have > >> some insight on this). I also see some complexity around this in the > >> security checks on marking shared writable mappings executable (e.g. in > >> mmap_violation_check()). > >> > >> In any case, it doesn't really make much sense to have a write-sealed > >> shared mapping, since you're essentially saying 'nothing _at all_ can write > >> to this' so it may as well be private. The semantics are unfortunate here, > >> the memory will still be shared read-only by MAP_PRIVATE mappings. > >> > >> A better choice here might be F_SEAL_FUTURE_WRITE (available from kernel > >>> =5.1) which does permit shared read-only mappings as this is explicitly > >> checked for in seal_check_future_write() invoked from shmem_mmap(). > >> > >> Regardless, I think the conclusion is that this is not a bug, but rather > >> that the documentation needs to be updated. > >> > > > > Adding docs people to cc list (sorry didn't think to do this in first > > reply).
