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authorWill Deacon <will@kernel.org>2020-06-16 19:03:49 +0100
committerWill Deacon <will@kernel.org>2020-06-17 11:18:58 +0100
commitb9249cba25a5dce5de87e5404503a5e11832c2dd (patch)
treebb83c56a535273b19fe4e033708cc754611d4d6c
parente575fb9e76c8e33440fb859572a8b7d430f053d6 (diff)
arm64: bti: Require clang >= 10.0.1 for in-kernel BTI support
Unfortunately, most versions of clang that support BTI are capable of miscompiling the kernel when converting a switch statement into a jump table. As an example, attempting to spawn a KVM guest results in a panic: [ 56.253312] Kernel panic - not syncing: bad mode [ 56.253834] CPU: 0 PID: 279 Comm: lkvm Not tainted 5.8.0-rc1 #2 [ 56.254225] Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015 [ 56.254712] Call trace: [ 56.254952] dump_backtrace+0x0/0x1d4 [ 56.255305] show_stack+0x1c/0x28 [ 56.255647] dump_stack+0xc4/0x128 [ 56.255905] panic+0x16c/0x35c [ 56.256146] bad_el0_sync+0x0/0x58 [ 56.256403] el1_sync_handler+0xb4/0xe0 [ 56.256674] el1_sync+0x7c/0x100 [ 56.256928] kvm_vm_ioctl_check_extension_generic+0x74/0x98 [ 56.257286] __arm64_sys_ioctl+0x94/0xcc [ 56.257569] el0_svc_common+0x9c/0x150 [ 56.257836] do_el0_svc+0x84/0x90 [ 56.258083] el0_sync_handler+0xf8/0x298 [ 56.258361] el0_sync+0x158/0x180 This is because the switch in kvm_vm_ioctl_check_extension_generic() is executed as an indirect branch to tail-call through a jump table: ffff800010032dc8: 3869694c ldrb w12, [x10, x9] ffff800010032dcc: 8b0c096b add x11, x11, x12, lsl #2 ffff800010032dd0: d61f0160 br x11 However, where the target case uses the stack, the landing pad is elided due to the presence of a paciasp instruction: ffff800010032e14: d503233f paciasp ffff800010032e18: a9bf7bfd stp x29, x30, [sp, #-16]! ffff800010032e1c: 910003fd mov x29, sp ffff800010032e20: aa0803e0 mov x0, x8 ffff800010032e24: 940017c0 bl ffff800010038d24 <kvm_vm_ioctl_check_extension> ffff800010032e28: 93407c00 sxtw x0, w0 ffff800010032e2c: a8c17bfd ldp x29, x30, [sp], #16 ffff800010032e30: d50323bf autiasp ffff800010032e34: d65f03c0 ret Unfortunately, this results in a fatal exception because paciasp is compatible only with branch-and-link (call) instructions and not simple indirect branches. A fix is being merged into Clang 10.0.1 so that a 'bti j' instruction is emitted as an explicit landing pad in this situation. Make in-kernel BTI depend on that compiler version when building with clang. Cc: Tom Stellard <tstellar@redhat.com> Cc: Daniel Kiss <daniel.kiss@arm.com> Reviewed-by: Mark Brown <broonie@kernel.org> Acked-by: Dave Martin <Dave.Martin@arm.com> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> Acked-by: Nick Desaulniers <ndesaulniers@google.com> Link: https://lore.kernel.org/r/20200615105524.GA2694@willie-the-truck Link: https://lore.kernel.org/r/20200616183630.2445-1-will@kernel.org Signed-off-by: Will Deacon <will@kernel.org>
-rw-r--r--arch/arm64/Kconfig2
1 files changed, 2 insertions, 0 deletions
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 31380da53689..4ae2419c14a8 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1630,6 +1630,8 @@ config ARM64_BTI_KERNEL
depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
depends on !CC_IS_GCC || GCC_VERSION >= 100100
+ # https://reviews.llvm.org/rGb8ae3fdfa579dbf366b1bb1cbfdbf8c51db7fa55
+ depends on !CC_IS_CLANG || CLANG_VERSION >= 100001
depends on !(CC_IS_CLANG && GCOV_KERNEL)
depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
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