Re: [PATCH] x86: remove the x32 syscall bitmask from syscall_get_nr()

From: Paul Moore
Date: Tue Feb 26 2013 - 15:58:33 EST

On Friday, February 15, 2013 12:21:43 PM 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.

I just wanted to check and see where things stood with this patch. I'm not
overly concerned about how this problem is solved, just that it is solved. If
someone else has a better approach that is fine with me; I'll even make the
offer to do additional testing if needed.


> Signed-off-by: Paul Moore <pmoore@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Will Drewry <wad@xxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> ---
> arch/x86/include/asm/syscall.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 1ace47b..2e188d6 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -29,13 +29,13 @@ extern const unsigned long sys_call_table[];
> */
> static inline int syscall_get_nr(struct task_struct *task, struct pt_regs
> *regs) {
> - return regs->orig_ax & __SYSCALL_MASK;
> + return regs->orig_ax;
> }
> static inline void syscall_rollback(struct task_struct *task,
> struct pt_regs *regs)
> {
> - regs->ax = regs->orig_ax & __SYSCALL_MASK;
> + regs->ax = regs->orig_ax;
> }
> static inline long syscall_get_error(struct task_struct *task,

paul moore
security and virtualization @ redhat

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