* Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> The syscall table base is a user controlled function pointer in kernel
> space. Use 'array_idx' to prevent any out of bounds speculation. While
> retpoline prevents speculating into a userspace directed target it does
> not stop the pointer de-reference, the concern is leaking memory
> relative to the syscall table base, by observing instruction cache
> behavior.
(The style problems/inconsistencies of the previous patches are repeated here too,
please fix.)
>
> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
> arch/x86/entry/common.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 03505ffbe1b6..f78bf8bfdfae 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -21,6 +21,7 @@
> #include <linux/export.h>
> #include <linux/context_tracking.h>
> #include <linux/user-return-notifier.h>
> +#include <linux/nospec.h>
> #include <linux/uprobes.h>
> #include <linux/livepatch.h>
> #include <linux/syscalls.h>
> @@ -284,6 +285,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
> * regs->orig_ax, which changes the behavior of some syscalls.
> */
> if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) {
> + nr = array_idx(nr & __SYSCALL_MASK, NR_syscalls);
> regs->ax = sys_call_table[nr & __SYSCALL_MASK](
> regs->di, regs->si, regs->dx,
> regs->r10, regs->r8, regs->r9);
Btw., in the future we could optimize the 64-bit fastpath here, by doing something
like:
if (unlikely(nr >= NR_syscalls)) {
nr = array_idx(nr, NR_syscalls);
...
} else {
if ((nr & __SYSCALL_MASK) < NR_syscalls) {
... X32 ABI ...
} else {
... error ...
}
}
This would remove 2-3 instructions from the 64-bit syscall fast-path I believe, by
pushing the x32 details to a slow-path.
But obviously that should not be part of the Spectre series.
Thanks,
Ingo
