From: Will Deacon will.deacon@arm.com
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 will.deacon@arm.com (cherry picked from commit 67ff2abfda2fdf2c2cc8f430e48e9c0df8996093) Signed-off-by: Alex Shi alex.shi@linaro.org --- arch/arm64/kernel/entry.S | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index 69e6d1a..074c402 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -902,6 +902,14 @@ __ni_sys_trace: .if \regsize == 64 msr tpidrro_el0, x30 // Restored in kernel_ventry .endif + /* + * Defend against branch aliasing attacks by pushing a dummy + * entry onto the return stack and using a RET instruction to + * enter the full-fat kernel vectors. + */ + bl 2f + b . +2: tramp_map_kernel x30 #ifdef CONFIG_RANDOMIZE_BASE adr x30, tramp_vectors + PAGE_SIZE @@ -913,7 +921,7 @@ __ni_sys_trace: msr vbar_el1, x30 add x30, x30, #(1b - tramp_vectors) isb - br x30 + ret .endm
.macro tramp_exit, regsize = 64