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From: Paul Moore <>
To: "H. Peter Anvin" <>
Subject: Re: [PATCH] x86: remove the x32 syscall bitmask from syscall_get_nr()
Date: Fri, 15 Feb 2013 15:52:26 -0500	[thread overview]
Message-ID: <2881752.183WHZTyfJ@sifl> (raw)
In-Reply-To: <>

On Friday, February 15, 2013 11:02:49 AM H. Peter Anvin wrote:
> On 02/15/2013 09:21 AM, Paul Moore wrote:
> > Commit fca460f95e928bae373daa8295877b6905bc62b8 simplified the x32
> > implementation by creating a syscall bitmask, equal to 0x40000000, that
> > could be applied to x32 syscalls such that the masked syscall number
> > would be the same as a x86_64 syscall.  While that patch was a nice
> > way to simplify the code, it went a bit too far by adding the mask to
> > syscall_get_nr(); returning the masked syscall numbers can cause
> > confusion with callers that expect syscall numbers matching the x32
> > ABI, e.g. unmasked syscall numbers.
> > 
> > This patch fixes this by simply removing the mask from syscall_get_nr()
> > while preserving the other changes from the original commit.  While
> > there are several syscall_get_nr() callers in the kernel, most simply
> > check that the syscall number is greater than zero, in this case this
> > patch will have no effect.  Of those remaining callers, they appear
> > to be few, seccomp and ftrace, and from my testing of seccomp without
> > this patch the original commit definitely breaks things; the seccomp
> > filter does not correctly filter the syscalls due to the difference in
> > syscall numbers in the BPF filter and the value from syscall_get_nr().
> > Applying this patch restores the seccomp BPF filter functionality on
> > x32.
> > 
> > I've tested this patch with the seccomp BPF filters as well as ftrace
> > and everything looks reasonable to me; needless to say general usage
> > seemed fine as well.
> Hi... it isn't 100% clear from the description if you have audited *all*
> the callers?

I audited all of the syscall_get_nr() callers using the LXR at with the 3.7 sources.  If you exclude all of the 
architecture dependent stuff that is non-x86 you arrive at the following list 
of callers:

* kernel/seccomp.c:seccomp_bpf_load()
This is where I noticed the problem, broken w/o the patch.

* lib/syscall.c:collect_syscall()/task_current_syscall()
The task_current_syscall() function is really only called by 
proc_pid_syscall() which displays the syscall number back to the user via a 
/proc entry, in which case this patch appears to correct a problem similar to 
what was seen with seccomp.

* kernel/trace/trace_syscalls.c:ftrace_syscall_enter()
* kernel/trace/trace_syscalls.c:ftrace_syscall_exit()
* kernel/trace/trace_syscalls.c:perf_syscall_enter()
* kernel/trace/trace_syscalls.c:perf_syscall_exit()
The ftrace/perf is the one user that I am least sure about, as noted above, I 
did some simple tests based on what I could find via Google but a quick review 
by someone who is more familiar with this code would be appreciated.  I'm most 
concerned about the syscall_metadata bits ...

* include/trace/events/syscall.h
Another, what I assume, is a ftrace user; I'm assuming the patch is reasonable 
based on my testing, but once again further review would be appreciated.

* arch/x86/kernel/ptrace.c:putreg32()
* arch/x86/kernel/signal.c:handle_signal()
* arch/x86/kernel/signal.c:do_signal()
Simple grater than zero checks.

paul moore
security and virtualization @ redhat

  reply	other threads:[~2013-02-15 20:52 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-02-15 17:21 Paul Moore
2013-02-15 19:02 ` H. Peter Anvin
2013-02-15 20:52   ` Paul Moore [this message]
2013-02-26 20:58 ` Paul Moore
2013-03-15 21:15   ` Paul Moore
2013-03-15 21:56     ` H. Peter Anvin
2013-03-15 22:18       ` H.J. Lu
2013-03-25 20:55         ` Paul Moore
2013-04-02 21:31           ` Paul Moore
2013-04-03  0:17 ` [tip:x86/urgent] " tip-bot for Paul Moore

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