Messages in this thread | | | Date | Thu, 28 Jun 2018 17:53:48 +0100 | From | Will Deacon <> | Subject | Re: [RFC PATCH for 4.18 2/2] rseq: check that rseq->rseq_cs padding is zero |
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Hi Mathieu,
On Thu, Jun 28, 2018 at 12:23:59PM -0400, Mathieu Desnoyers wrote: > On 32-bit kernels, the rseq->rseq_cs_padding field is never read by the > kernel. However, 64-bit kernels dealing with 32-bit compat tasks read the > full 64-bit in its entirety, and terminates the offending process with > a segmentation fault if the upper 32 bits are set due to failure of > copy_from_user(). > > Ensure that both 32-bit and 64-bit kernels dealing with 32-bit tasks end > up terminating offending tasks with a segmentation fault if the upper > 32-bit padding bits (rseq->rseq_cs_padding) are set by adding an explicit > check that padding is zero on 32-bit kernels. > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > CC: Peter Zijlstra <peterz@infradead.org> > CC: Paul Turner <pjt@google.com> > CC: Thomas Gleixner <tglx@linutronix.de> > CC: Andy Lutomirski <luto@amacapital.net> > CC: Andi Kleen <andi@firstfloor.org> > CC: Dave Watson <davejwatson@fb.com> > CC: Chris Lameter <cl@linux.com> > CC: Ingo Molnar <mingo@redhat.com> > CC: "H. Peter Anvin" <hpa@zytor.com> > CC: Ben Maurer <bmaurer@fb.com> > CC: Steven Rostedt <rostedt@goodmis.org> > CC: Josh Triplett <josh@joshtriplett.org> > CC: Linus Torvalds <torvalds@linux-foundation.org> > CC: Andrew Morton <akpm@linux-foundation.org> > CC: Russell King <linux@arm.linux.org.uk> > CC: Catalin Marinas <catalin.marinas@arm.com> > CC: Will Deacon <will.deacon@arm.com> > CC: Michael Kerrisk <mtk.manpages@gmail.com> > CC: Boqun Feng <boqun.feng@gmail.com> > CC: linux-api@vger.kernel.org > --- > kernel/rseq.c | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/kernel/rseq.c b/kernel/rseq.c > index 4ba582046fcd..b038f35a60d6 100644 > --- a/kernel/rseq.c > +++ b/kernel/rseq.c > @@ -112,6 +112,29 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t) > return 0; > } > > +#ifndef __LP64__ > +/* > + * Ensure that padding is zero. > + */ > +static int check_rseq_cs_padding(struct task_struct *t) > +{ > + unsigned long pad; > + int ret; > + > + ret = __get_user(pad, &t->rseq->rseq_cs_padding); > + if (ret) > + return ret; > + if (pad) > + return -EFAULT; > + return 0; > +} > +#else > +static int check_rseq_cs_padding(struct task_struct *t) > +{ > + return 0; > +} > +#endif
I'm still not sure how this works with a 64-bit kernel and a compat (32-bit) task. The check_rseq_cs_padding() will return 0 regardless of the upper bits of the rseq_cs field, whereas a native 32-bit kernel would actually go and check them.
What am I missing here?
Will
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