Hi Ard, On Thu, Jan 04, 2018 at 04:24:22PM +0000, Ard Biesheuvel wrote: > On 4 January 2018 at 15:08, Will Deacon <[email protected]> wrote: > > Speculation attacks against the entry trampoline can potentially resteer > > the speculative instruction stream through the indirect branch and into > > arbitrary gadgets within the kernel. > > > > This patch defends against these attacks by forcing a misprediction > > through the return stack: a dummy BL instruction loads an entry into > > the stack, so that the predicted program flow of the subsequent RET > > instruction is to a branch-to-self instruction which is finally resolved > > as a branch to the kernel vectors with speculation suppressed. > > > > Signed-off-by: Will Deacon <[email protected]> > > --- > > arch/arm64/kernel/entry.S | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > > index 031392ee5f47..b9feb587294d 100644 > > --- a/arch/arm64/kernel/entry.S > > +++ b/arch/arm64/kernel/entry.S > > @@ -1029,6 +1029,9 @@ alternative_else_nop_endif > > .if \regsize == 64 > > msr tpidrro_el0, x30 // Restored in kernel_ventry > > .endif > > + bl 2f > > + b . > > +2: > > This deserves a comment, I guess?
Yeah, I suppose ;) I'll lift something out of the commit message. > Also, is deliberately unbalancing the return stack likely to cause > performance problems, e.g., in libc hot paths? I don't think so, because it remains balanced after this code. We push an entry on with the BL and pop it with the RET; the rest of the return stack remains unchanged. That said, I'm also not sure what we could do differently here! Will
