* Brian Gerst <[email protected]> wrote:
> Both the execve and sigreturn family of syscalls have the ability to change
> registers in ways that may not be compatabile with the syscall path they
> were called from. In particular, sysret and sysexit can't handle non-default
> %cs and %ss, and some bits in eflags. These syscalls have stubs that are
> hardcoded to jump to the iret path, and not return to the original syscall
> path. Commit 76f5df43cab5e765c0bd42289103e8f625813ae1 (Always allocate a
> complete "struct pt_regs" on the kernel stack) recently changed this for
> some 32-bit compat syscalls, but introduced a bug where execve from a 32-bit
> program to a 64-bit program would fail because it still returned via sysretl.
> This caused Wine to fail when built for both 32-bit and 64-bit.
>
> This patch sets TIF_NOTIFY_RESUME for execve and sigreturn so that the iret
> path is always taken on exit to userspace.
>
> Signed-off-by: Brian Gerst <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Denys Vlasenko <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: H. Peter Anvin <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> ---
> arch/x86/ia32/ia32_signal.c | 2 ++
> arch/x86/include/asm/ptrace.h | 2 +-
> arch/x86/include/asm/thread_info.h | 7 +++++++
> arch/x86/kernel/process_32.c | 6 +-----
> arch/x86/kernel/process_64.c | 1 +
> arch/x86/kernel/signal.c | 2 ++
> 6 files changed, 14 insertions(+), 6 deletions(-)
Applied the fix to tip:x86/asm, thanks Brian!
> +
> +/*
> + * force syscall return via iret by making it look as if there was
> + * some work pending.
> +*/
> +#define force_iret() set_thread_flag(TIF_NOTIFY_RESUME)
I extended this comment to:
/*
* Force syscall return via IRET by making it look as if there was
* some work pending. IRET is our most capable (but slowest) syscall
* return path, which is able to restore modified SS, CS and certain
* EFLAGS values that other (fast) syscall return instructions
* are not able to restore properly.
*/
#define force_iret() set_thread_flag(TIF_NOTIFY_RESUME)
Just to preserve the underlying reason for force_iret() for the future
and such.
Btw., it might be a worthwile optimization to detect non-standard SS,
CS and EFLAGS values and only force_iret() in that case, that will
speed up 99.9999% of execve() and sigreturn() syscalls and only force
the 'weird' process startup modes into the slow return path.
>From an access security POV it should be a relatively safe
optimization: if we get it wrong then we don't allow certain ABI
angles, but we won't make the kernel unsafe AFAICS.
Thanks,
Ingo
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