Sami reports that linux panic()s when resuming from suspend to RAM. This is because when CPUs are brought back online, they re-enable any necessary mitigations.
The Spectre-v2 and Spectre-BHB mitigations interact as both need to done by KVM when exiting a guest. Slots KVM can use as vectors are allocated, and templates for the mitigation are patched into the vector.
This fails if a new slot needs to be allocated once the kernel has finished booting as it is no-longer possible to modify KVM's vectors: | root@adam:/sys/devices/system/cpu/cpu1# echo 1 > online | Unable to handle kernel write to read-only memory at virtual add> | Mem abort info: | ESR = 0x9600004e | Exception class = DABT (current EL), IL = 32 bits | SET = 0, FnV = 0 | EA = 0, S1PTW = 0 | Data abort info: | ISV = 0, ISS = 0x0000004e | CM = 0, WnR = 1 | swapper pgtable: 4k pages, 48-bit VAs, pgdp = 000000000f07a71c | [ffff800000b4b800] pgd=00000009ffff8803, pud=00000009ffff7803, p> | Internal error: Oops: 9600004e [#1] PREEMPT SMP | Modules linked in: | Process swapper/1 (pid: 0, stack limit = 0x0000000063153c53) | CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.252-dirty #14 | Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno De> | pstate: 000001c5 (nzcv dAIF -PAN -UAO) | pc : __memcpy+0x48/0x180 | lr : __copy_hyp_vect_bpi+0x64/0x90
| Call trace: | __memcpy+0x48/0x180 | kvm_setup_bhb_slot+0x204/0x2a8 | spectre_bhb_enable_mitigation+0x1b8/0x1d0 | __verify_local_cpu_caps+0x54/0xf0 | check_local_cpu_capabilities+0xc4/0x184 | secondary_start_kernel+0xb0/0x170 | Code: b8404423 b80044c3 36180064 f8408423 (f80084c3) | ---[ end trace 859bcacb09555348 ]--- | Kernel panic - not syncing: Attempted to kill the idle task! | SMP: stopping secondary CPUs | Kernel Offset: disabled | CPU features: 0x10,25806086 | Memory Limit: none | ---[ end Kernel panic - not syncing: Attempted to kill the idle ]
This is only a problem on platforms where there is only one CPU that is vulnerable to both Spectre-v2 and Spectre-BHB.
The Spectre-v2 mitigation identifies the slot it can re-use by the CPU's 'fn'. It unconditionally writes the slot number and 'template_start' pointer. The Spectre-BHB mitigation identifies slots it can re-use by the CPU's template_start pointer, which was previously clobbered by the Spectre-v2 mitigation.
When there is only one CPU that is vulnerable to both issues, this causes Spectre-v2 to try to allocate a new slot, which fails.
Change both mitigations to check whether they are changing the slot this CPU uses before writing the percpu variables again.
This issue only exists in the stable backports for Spectre-BHB which have to use totally different infrastructure to mainline.
Reported-by: Sami Lee sami.lee@mediatek.com Fixes: c20d55174479 ("arm64: Mitigate spectre style branch history side channels") Signed-off-by: James Morse james.morse@arm.com --- arch/arm64/kernel/cpu_errata.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c index 5435550d1c9b..9418d2257b9a 100644 --- a/arch/arm64/kernel/cpu_errata.c +++ b/arch/arm64/kernel/cpu_errata.c @@ -142,9 +142,12 @@ static void install_bp_hardening_cb(bp_hardening_cb_t fn, __copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end); }
- __this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot); - __this_cpu_write(bp_hardening_data.fn, fn); - __this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start); + if (fn != __this_cpu_read(bp_hardening_data.fn)) { + __this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot); + __this_cpu_write(bp_hardening_data.fn, fn); + __this_cpu_write(bp_hardening_data.template_start, + hyp_vecs_start); + } spin_unlock(&bp_lock); } #else @@ -1203,8 +1206,11 @@ static void kvm_setup_bhb_slot(const char *hyp_vecs_start) __copy_hyp_vect_bpi(slot, hyp_vecs_start, hyp_vecs_end); }
- __this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot); - __this_cpu_write(bp_hardening_data.template_start, hyp_vecs_start); + if (hyp_vecs_start != __this_cpu_read(bp_hardening_data.template_start)) { + __this_cpu_write(bp_hardening_data.hyp_vectors_slot, slot); + __this_cpu_write(bp_hardening_data.template_start, + hyp_vecs_start); + } spin_unlock(&bp_lock); } #else