On Tue, Dec 12, 2017 at 11:31:02AM +0100, Ingo Molnar wrote: > > * Thomas Gleixner <t...@linutronix.de> wrote: > > > On Fri, 8 Dec 2017, Darren Hart wrote: > > > > > From: "Darren Hart (VMware)" <dvh...@infradead.org> > > > > > > uaddr alignment is currently tested by get_futex_key(). We can catch > > > misalignment earlier in sys_futex and return -EINVAL sooner. This > > > simplifies get_futex_key() a little, but more importantly exits the > > > kernel as soon as an invalid parameter is detected. > > > > > > Passes all selftests/futex testcases on a dual socket Xeon E5-2670, 16 > > > physical cores total, 32 threads total. > > > > > > Signed-off-by: Darren Hart (VMware) <dvh...@infradead.org> > > > Cc: Thomas Gleixner <t...@linutronix.de> > > > Cc: Ingo Molnar <mi...@redhat.com> > > > Cc: Peter Zijlstra <pet...@infradead.org> > > > Cc: Darren Hart <dvh...@infradead.org> > > > --- > > > kernel/futex.c | 7 +++++-- > > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > > > diff --git a/kernel/futex.c b/kernel/futex.c > > > index 76ed592..c3ee6c4 100644 > > > --- a/kernel/futex.c > > > +++ b/kernel/futex.c > > > @@ -509,8 +509,6 @@ get_futex_key(u32 __user *uaddr, int fshared, union > > > futex_key *key, int rw) > > > * The futex address must be "naturally" aligned. > > > */ > > > key->both.offset = address % PAGE_SIZE; > > > - if (unlikely((address % sizeof(u32)) != 0)) > > > - return -EINVAL; > > > address -= key->both.offset; > > > > > > if (unlikely(!access_ok(rw, uaddr, sizeof(u32)))) > > > @@ -3525,6 +3523,11 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, > > > op, u32, val, > > > u32 val2 = 0; > > > int cmd = op & FUTEX_CMD_MASK; > > > > > > + /* Only allow for aligned uaddr variables */ > > > + if (unlikely((unsigned long)uaddr % sizeof(u32) != 0 || > > > + (unsigned long)uaddr2 % sizeof(u32) != 0)) > > > > Errm. How is that supposed to work? uaddr2 is not used by all opcodes..... > > So to explain the curious timing of the mails from Thomas and me: I told > Thomas > about the breakage over IRC and he found the likely bug! ;-)
My thinking had been that while uaddr2 is not use by all the op-codes, it is not ever used as anything but a userspace address and when it isn't used, it shouldn't be getting garbage passed in. So if it's failing due to a uaddr2 not being aligned for an op-code that doesn't use it... that would have to be on FUTEX_WAIT*, FUTEX_WAKE, FUTEX_WAKE_BITSET, or FUTEX_*_PI. If it's failing on distro boot, the PI ops are unlikely candidates. I'm curious what a valid use of this would be, and why my own tests didn't catch it. I can move the uaddr2 test under a conditional on the cmd for now. Would that be acceptable? Then I can add a WARNON for the other ops for my own testing, but not for upstream. -- Darren Hart VMware Open Source Technology Center
