From: Mark Rutland mark.rutland@arm.com
commit 35b6b28e69985eafb20b3b2c7bd6eca452b56b53 upstream.
When branch target identifiers are in use, code reachable via an indirect branch requires a BTI landing pad at the branch target site.
When building FTRACE_WITH_REGS atop patchable-function-entry, we miss BTIs at the start start of the `ftrace_caller` and `ftrace_regs_caller` trampolines, and when these are called from a module via a PLT (which will use a `BR X16`), we will encounter a BTI failure, e.g.
| # insmod lkdtm.ko | lkdtm: No crash points registered, enable through debugfs | # echo function_graph > /sys/kernel/debug/tracing/current_tracer | # cat /sys/kernel/debug/provoke-crash/DIRECT | Unhandled 64-bit el1h sync exception on CPU0, ESR 0x34000001 -- BTI | CPU: 0 PID: 174 Comm: cat Not tainted 5.16.0-rc2-dirty #3 | Hardware name: linux,dummy-virt (DT) | pstate: 60400405 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=jc) | pc : ftrace_caller+0x0/0x3c | lr : lkdtm_debugfs_open+0xc/0x20 [lkdtm] | sp : ffff800012e43b00 | x29: ffff800012e43b00 x28: 0000000000000000 x27: ffff800012e43c88 | x26: 0000000000000000 x25: 0000000000000000 x24: ffff0000c171f200 | x23: ffff0000c27b1e00 x22: ffff0000c2265240 x21: ffff0000c23c8c30 | x20: ffff8000090ba380 x19: 0000000000000000 x18: 0000000000000000 | x17: 0000000000000000 x16: ffff80001002bb4c x15: 0000000000000000 | x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000900ff0 | x11: ffff0000c4166310 x10: ffff800012e43b00 x9 : ffff8000104f2384 | x8 : 0000000000000001 x7 : 0000000000000000 x6 : 000000000000003f | x5 : 0000000000000040 x4 : ffff800012e43af0 x3 : 0000000000000001 | x2 : ffff8000090b0000 x1 : ffff0000c171f200 x0 : ffff0000c23c8c30 | Kernel panic - not syncing: Unhandled exception | CPU: 0 PID: 174 Comm: cat Not tainted 5.16.0-rc2-dirty #3 | Hardware name: linux,dummy-virt (DT) | Call trace: | dump_backtrace+0x0/0x1a4 | show_stack+0x24/0x30 | dump_stack_lvl+0x68/0x84 | dump_stack+0x1c/0x38 | panic+0x168/0x360 | arm64_exit_nmi.isra.0+0x0/0x80 | el1h_64_sync_handler+0x68/0xd4 | el1h_64_sync+0x78/0x7c | ftrace_caller+0x0/0x3c | do_dentry_open+0x134/0x3b0 | vfs_open+0x38/0x44 | path_openat+0x89c/0xe40 | do_filp_open+0x8c/0x13c | do_sys_openat2+0xbc/0x174 | __arm64_sys_openat+0x6c/0xbc | invoke_syscall+0x50/0x120 | el0_svc_common.constprop.0+0xdc/0x100 | do_el0_svc+0x84/0xa0 | el0_svc+0x28/0x80 | el0t_64_sync_handler+0xa8/0x130 | el0t_64_sync+0x1a0/0x1a4 | SMP: stopping secondary CPUs | Kernel Offset: disabled | CPU features: 0x0,00000f42,da660c5f | Memory Limit: none | ---[ end Kernel panic - not syncing: Unhandled exception ]---
Fix this by adding the required `BTI C`, as we only require these to be reachable via BL for direct calls or BR X16/X17 for PLTs. For now, these are open-coded in the function prologue, matching the style of the `__hwasan_tag_mismatch` trampoline.
In future we may wish to consider adding a new SYM_CODE_START_*() variant which has an implicit BTI.
When ftrace is built atop mcount, the trampolines are marked with SYM_FUNC_START(), and so get an implicit BTI. We may need to change these over to SYM_CODE_START() in future for RELIABLE_STACKTRACE, in case we need to apply special care aroud the return address being rewritten.
Fixes: 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") Signed-off-by: Mark Rutland mark.rutland@arm.com Cc: Catalin Marinas catalin.marinas@arm.com Cc: Mark Brown broonie@kernel.org Cc: Will Deacon will@kernel.org Reviewed-by: Mark Brown broonie@kernel.org Link: https://lore.kernel.org/r/20211129135709.2274019-1-mark.rutland@arm.com Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/arm64/kernel/entry-ftrace.S | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/arch/arm64/kernel/entry-ftrace.S +++ b/arch/arm64/kernel/entry-ftrace.S @@ -77,11 +77,17 @@ .endm
SYM_CODE_START(ftrace_regs_caller) +#ifdef BTI_C + BTI_C +#endif ftrace_regs_entry 1 b ftrace_common SYM_CODE_END(ftrace_regs_caller)
SYM_CODE_START(ftrace_caller) +#ifdef BTI_C + BTI_C +#endif ftrace_regs_entry 0 b ftrace_common SYM_CODE_END(ftrace_caller)