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x86/speculation: Protect against userspace-userspace spectreRSB
The article "Spectre Returns! Speculation Attacks using the Return Stack Buffer"  describes two new (sub-)variants of spectrev2-like attacks, making use solely of the RSB contents even on CPUs that don't fallback to BTB on RSB underflow (Skylake+). Mitigate userspace-userspace attacks by always unconditionally filling RSB on context switch when the generic spectrev2 mitigation has been enabled.  https://arxiv.org/pdf/1807.07940.pdf Signed-off-by: Jiri Kosina <email@example.com> Signed-off-by: Thomas Gleixner <firstname.lastname@example.org> Reviewed-by: Josh Poimboeuf <email@example.com> Acked-by: Tim Chen <firstname.lastname@example.org> Cc: Konrad Rzeszutek Wilk <email@example.com> Cc: Borislav Petkov <firstname.lastname@example.org> Cc: David Woodhouse <email@example.com> Cc: Peter Zijlstra <firstname.lastname@example.org> Cc: Linus Torvalds <email@example.com> Cc: firstname.lastname@example.org Link: https://lkml.kernel.org/r/nycvar.YFH.email@example.com
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