[PATCH 5.19 368/717] arm64: ftrace: fix module PLTs with mcount

Greg Kroah-Hartman
2022-10-22T08:07:35Z

From: Mark Rutland <mark.rutland@arm.com>

[ Upstream commit 8cfb08575c6d4585f1ce0deeb189e5c824776b04 ]

Li Huafei reports that mcount-based ftrace with module PLTs was broken
by commit:

a6253579977e4c6f ("arm64: ftrace: consistently handle PLTs.")

When a module PLTs are used and a module is loaded sufficiently far away
from the kernel, we'll create PLTs for any branches which are
out-of-range. These are separate from the special ftrace trampoline
PLTs, which the module PLT code doesn't directly manipulate.

When mcount is in use this is a problem, as each mcount callsite in a
module will be initialized to point to a module PLT, but since commit
a6253579977e4c6f ftrace_make_nop() will assume that the callsite has
been initialized to point to the special ftrace trampoline PLT, and
ftrace_find_callable_addr() rejects other cases.

This means that when ftrace tries to initialize a callsite via
ftrace_make_nop(), the call to ftrace_find_callable_addr() will find
that the `_mcount` stub is out-of-range and is not handled by the ftrace
PLT, resulting in a splat:

| ftrace_test: loading out-of-tree module taints kernel.
| ftrace: no module PLT for _mcount
| ------------[ ftrace bug ]------------
| ftrace failed to modify
| [<ffff800029180014>] 0xffff800029180014
| actual: 44:00:00:94
| Initializing ftrace call sites
| ftrace record flags: 2000000
| (0)
| expected tramp: ffff80000802eb3c
| ------------[ cut here ]------------
| WARNING: CPU: 3 PID: 157 at kernel/trace/ftrace.c:2120 ftrace_bug+0x94/0x270
| Modules linked in:
| CPU: 3 PID: 157 Comm: insmod Tainted: G O 6.0.0-rc6-00151-gcd722513a189-dirty #22
| Hardware name: linux,dummy-virt (DT)
| pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| pc : ftrace_bug+0x94/0x270
| lr : ftrace_bug+0x21c/0x270
| sp : ffff80000b2bbaf0
| x29: ffff80000b2bbaf0 x28: 0000000000000000 x27: ffff0000c4d38000
| x26: 0000000000000001 x25: ffff800009d7e000 x24: ffff0000c4d86e00
| x23: 0000000002000000 x22: ffff80000a62b000 x21: ffff8000098ebea8
| x20: ffff0000c4d38000 x19: ffff80000aa24158 x18: ffffffffffffffff
| x17: 0000000000000000 x16: 0a0d2d2d2d2d2d2d x15: ffff800009aa9118
| x14: 0000000000000000 x13: 6333626532303830 x12: 3030303866666666
| x11: 203a706d61727420 x10: 6465746365707865 x9 : 3362653230383030
| x8 : c0000000ffffefff x7 : 0000000000017fe8 x6 : 000000000000bff4
| x5 : 0000000000057fa8 x4 : 0000000000000000 x3 : 0000000000000001
| x2 : ad2cb14bb5438900 x1 : 0000000000000000 x0 : 0000000000000022
| Call trace:
| ftrace_bug+0x94/0x270
| ftrace_process_locs+0x308/0x430
| ftrace_module_init+0x44/0x60
| load_module+0x15b4/0x1ce8
| __do_sys_init_module+0x1ec/0x238
| __arm64_sys_init_module+0x24/0x30
| invoke_syscall+0x54/0x118
| el0_svc_common.constprop.4+0x84/0x100
| do_el0_svc+0x3c/0xd0
| el0_svc+0x1c/0x50
| el0t_64_sync_handler+0x90/0xb8
| el0t_64_sync+0x15c/0x160
| ---[ end trace 0000000000000000 ]---
| ---------test_init-----------

Fix this by reverting to the old behaviour of ignoring the old
instruction when initialising an mcount callsite in a module, which was
the behaviour prior to commit a6253579977e4c6f.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: a6253579977e ("arm64: ftrace: consistently handle PLTs.")
Reported-by: Li Huafei <lihuafei1@huawei.com>
Link: https://lore.kernel.org/linux-arm-kernel/20220929094134.99512-1-lihuafei1@huawei.com
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220929134525.798593-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm64/kernel/ftrace.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index ea5dc7c90f46..b49ba9a24bcc 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -217,11 +217,26 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
unsigned long pc = rec->ip;
u32 old = 0, new;

+ new = aarch64_insn_gen_nop();
+
+ /*
+ * When using mcount, callsites in modules may have been initalized to
+ * call an arbitrary module PLT (which redirects to the _mcount stub)
+ * rather than the ftrace PLT we'll use at runtime (which redirects to
+ * the ftrace trampoline). We can ignore the old PLT when initializing
+ * the callsite.
+ *
+ * Note: 'mod' is only set at module load time.
+ */
+ if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) &&
+ IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) && mod) {
+ return aarch64_insn_patch_text_nosync((void *)pc, new);
+ }
+
if (!ftrace_find_callable_addr(rec, mod, &addr))
return -EINVAL;

old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
- new = aarch64_insn_gen_nop();

return ftrace_modify_code(pc, old, new, true);
}
--
2.35.1