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arm64: Mitigate spectre style branch history side channels

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Speculation attacks against some high-performance processors can
make use of branch history to influence future speculation.
When taking an exception from user-space, a sequence of branches
or a firmware call overwrites or invalidates the branch history.

The sequence of branches is added to the vectors, and should appear
before the first indirect branch. For systems using KPTI the sequence
is added to the kpti trampoline where it has a free register as the exit
from the trampoline is via a 'ret'. For systems not using KPTI, the same
register tricks are used to free up a register in the vectors.

For the firmware call, arch-workaround-3 clobbers 4 registers, so
there is no choice but to save them to the EL1 stack. This only happens
for entry from EL0, so if we take an exception due to the stack access,
it will not become re-entrant.

For KVM, the existing branch-predictor-hardening vectors are used.
When a spectre version of these vectors is in use, the firmware call
is sufficient to mitigate against Spectre-BHB. For the non-spectre
versions, the sequence of branches is added to the indirect vector.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
This commit is contained in:
James Morse
2021-11-10 14:48:00 +00:00
parent dee435be76
commit 558c303c97
12 changed files with 352 additions and 2 deletions
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14
arch/arm64/include/asm/assembler.h
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@@ -852,7 +852,9 @@ alternative_endif
.macro __mitigate_spectre_bhb_loop tmp
#ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
mov \tmp, #32
alternative_cb spectre_bhb_patch_loop_iter
mov \tmp, #32 // Patched to correct the immediate
alternative_cb_end
.Lspectre_bhb_loop\@:
b . + 4
subs \tmp, \tmp, #1
@@ -861,6 +863,16 @@ alternative_endif
#endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
.endm
.macro mitigate_spectre_bhb_loop tmp
#ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
alternative_cb spectre_bhb_patch_loop_mitigation_enable
b .L_spectre_bhb_loop_done\@ // Patched to NOP
alternative_cb_end
__mitigate_spectre_bhb_loop \tmp
.L_spectre_bhb_loop_done\@:
#endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
.endm
/* Save/restores x0-x3 to the stack */
.macro __mitigate_spectre_bhb_fw
#ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY