Re: [PATCH 3.18 055/104] arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value

Greg Kroah-Hartman
2019-04-25T07:51:11Z

On Wed, Apr 24, 2019 at 10:35:20AM -0700, Nathan Chancellor wrote:
> Hi Greg,
>
> On Wed, Apr 24, 2019 at 07:09:12PM +0200, Greg Kroah-Hartman wrote:
> > From: Will Deacon <will.deacon@arm.com>
> >
> > commit 045afc24124d80c6998d9c770844c67912083506 upstream.
> >
> > Rather embarrassingly, our futex() FUTEX_WAKE_OP implementation doesn't
> > explicitly set the return value on the non-faulting path and instead
> > leaves it holding the result of the underlying atomic operation. This
> > means that any FUTEX_WAKE_OP atomic operation which computes a non-zero
> > value will be reported as having failed. Regrettably, I wrote the buggy
> > code back in 2011 and it was upstreamed as part of the initial arm64
> > support in 2012.
> >
> > The reasons we appear to get away with this are:
> >
> > 1. FUTEX_WAKE_OP is rarely used and therefore doesn't appear to get
> > exercised by futex() test applications
> >
> > 2. If the result of the atomic operation is zero, the system call
> > behaves correctly
> >
> > 3. Prior to version 2.25, the only operation used by GLIBC set the
> > futex to zero, and therefore worked as expected. From 2.25 onwards,
> > FUTEX_WAKE_OP is not used by GLIBC at all.
> >
> > Fix the implementation by ensuring that the return value is either 0
> > to indicate that the atomic operation completed successfully, or -EFAULT
> > if we encountered a fault when accessing the user mapping.
> >
> > Cc: <stable@kernel.org>
> > Fixes: 6170a97460db ("arm64: Atomic operations")
> > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >
> > ---
> > arch/arm64/include/asm/futex.h | 16 ++++++++--------
> > 1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > --- a/arch/arm64/include/asm/futex.h
> > +++ b/arch/arm64/include/asm/futex.h
> > @@ -26,8 +26,8 @@
> > asm volatile( \
> > "1: ldxr %w1, %2\n" \
> > insn "\n" \
> > -"2: stlxr %w3, %w0, %2\n" \
> > -" cbnz %w3, 1b\n" \
> > +"2: stlxr %w0, %w3, %2\n" \
> > +" cbnz %w0, 1b\n" \
> > " dmb ish\n" \
> > "3:\n" \
> > " .pushsection .fixup,\"ax\"\n" \
> > @@ -50,7 +50,7 @@ futex_atomic_op_inuser(unsigned int enco
> > int cmp = (encoded_op >> 24) & 15;
> > int oparg = (int)(encoded_op << 8) >> 20;
> > int cmparg = (int)(encoded_op << 20) >> 20;
> > - int oldval = 0, ret, tmp;
> > + int oldval, ret, tmp;
>
> Please ensure the follow up fix gets queued up for 3.18 as well
> (backport attached).

Ugh, I forgot it, thanks for the backport, now queued up.

greg k-h