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: 20708c10e051 ("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>
projects / kernel.git / commitdiff