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

[Greg Kroah-Hartman]

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@xxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Fixes: 6170a97460db ("arm64: Atomic operations")
> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > 
> > ---
> >  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