On Fri, Dec 14, 2012 at 08:12:45AM -0800, Andy Lutomirski wrote: > On Fri, Dec 14, 2012 at 6:49 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > On Fri, Dec 14, 2012 at 03:14:50AM -0800, Andy Lutomirski wrote: > > > >> > Wait a minute. get_user_pages() relies on ->mmap_sem being held. Unless > >> > I'm seriously misreading your patch it removes that protection. And yes, > >> > I'm aware of execve-related exception; it's in special circumstances - > >> > bprm->mm is guaranteed to be not shared (and we need to rearchitect that > >> > area anyway, but that's a separate story). > >> > >> Unless I completely screwed up the patch, ->mmap_sem is still held for > >> read (it's downgraded from write). It's just not held for write > >> anymore. > > > > Huh? I'm talking about the call of get_user_pages() in aio_setup_ring(). > > With your patch it's done completely outside of ->mmap_sem, isn't it? > > Oh, /that/ call to get_user_pages. That would qualify as screwing up... > > Since dropping and reacquiring mmap_sem there is probably a bad idea > there, I'll rework this and post a v2. FWIW, I've done some checking of ->mmap_sem uses yesterday. Got further than the last time; catch so far, just from find_vma() audit: * arm swp_emulate.c - missing ->mmap_sem around find_vma(). Fix sent to rmk. * blackfin ptrace - find_vma() without any protection, definitely broken * m68k sys_cacheflush() - ditto * mips process_fpemu_return() - ditto * mips octeon_flush_cache_sigtramp() - ditto * omap_vout_uservirt_to_phys() - ditto, patch sent * vb2_get_contig_userptr() - probaly a bug, unless I've misread the (very twisty maze of) v4l2 code leading to it * vb2_get_contig_userptr() - ditto * gntdev_ioctl_get_offset_for_vaddr() - definitely broken and there's a couple of dubious places in arch/* I hadn't finished with, plus a lot in mm/* proper. That's just from a couple of days of RTFS. The locking in there is far too convoluted as it is; worse, it's not localized code-wise, so rechecking correctness is going to remain a big time-sink ;-/ Making it *more* complex doesn't look like a good idea, TBH... BTW, the __get_user_pages()/find_extend_vma()/mlock_vma_pages_range() pile is really asking for trouble; sure, the recursion there is limited, but it deserves a comment. Moreover, the damn thing is reachable from coredump path and there we do *not* have ->mmap_sem held. We don't reach the VM_BUG_ON() in __mlock_vma_pages_range(), but the reason for that also deserves a comment, IMO. Moreover, I'm not quite convinced that huge_memory.c and ksm.c can't run into all kinds of interesting races with ongoing coredump. Looking into it... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>