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arm64: use RET instruction for exiting the trampoline

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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>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
This commit is contained in:
Will Deacon
2017-11-14 16:15:59 +00:00
committed by Greg Kroah-Hartman
parent 06fe41f852
commit 753593688b
1 changed files with 4 additions and 1 deletions
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5
arch/arm64/kernel/entry.S
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@@ -948,6 +948,9 @@ __ni_sys_trace:
.if \regsize == 64
msr tpidrro_el0, x30 // Restored in kernel_ventry
.endif
bl 2f
b .
2:
tramp_map_kernel x30
#ifdef CONFIG_RANDOMIZE_BASE
adr x30, tramp_vectors + PAGE_SIZE
@@ -960,7 +963,7 @@ alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
msr vbar_el1, x30
add x30, x30, #(1b - tramp_vectors)
isb
br x30
ret
.endm
.macro tramp_exit, regsize = 64