[PATCH -tip v10 14/16] x86/unwind: Recover kretprobe trampoline entry

Masami Hiramatsu
2021-07-29T14:08:07Z

Since the kretprobe replaces the function return address with
the kretprobe_trampoline on the stack, x86 unwinders can not
continue the stack unwinding at that point, or record
kretprobe_trampoline instead of correct return address.

To fix this issue, find the correct return address from task's
kretprobe_instances as like as function-graph tracer does.

With this fix, the unwinder can correctly unwind the stack
from kretprobe event on x86, as below.

<...>-135 [003] ...1 6.722338: r_full_proxy_read_0: (vfs_read+0xab/0x1a0 <- full_proxy_read)
<...>-135 [003] ...1 6.722377: <stack trace>
=> kretprobe_trace_func+0x209/0x2f0
=> kretprobe_dispatcher+0x4a/0x70
=> __kretprobe_trampoline_handler+0xca/0x150
=> trampoline_handler+0x44/0x70
=> kretprobe_trampoline+0x2a/0x50
=> vfs_read+0xab/0x1a0
=> ksys_read+0x5f/0xe0
=> do_syscall_64+0x33/0x40
=> entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
Changes in v9:
- Update comment so that it explains why the strange address passed
to unwind_recover_kretprobe().
Changes in v7:
- Remove superfluous #include <linux/kprobes.h>.
Changes in v5:
- Fix the case of interrupt happens on kretprobe_trampoline+0.
Changes in v3:
- Split out the kretprobe side patch
- Fix build error when CONFIG_KRETPROBES=n.
Changes in v2:
- Remove kretprobe wrapper functions from unwind_orc.c
- Do not fixup state->ip when unwinding with regs because
kretprobe fixup instruction pointer before calling handler.
---
arch/x86/include/asm/unwind.h | 23 +++++++++++++++++++++++
arch/x86/kernel/unwind_frame.c | 3 +--
arch/x86/kernel/unwind_guess.c | 3 +--
arch/x86/kernel/unwind_orc.c | 21 +++++++++++++++++----
4 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index 70fc159ebe69..fca2e783e3ce 100644
--- a/arch/x86/include/asm/unwind.h
+++ b/arch/x86/include/asm/unwind.h
@@ -4,6 +4,7 @@

#include <linux/sched.h>
#include <linux/ftrace.h>
+#include <linux/kprobes.h>
#include <asm/ptrace.h>
#include <asm/stacktrace.h>

@@ -15,6 +16,7 @@ struct unwind_state {
unsigned long stack_mask;
struct task_struct *task;
int graph_idx;
+ struct llist_node *kr_cur;
bool error;
#if defined(CONFIG_UNWINDER_ORC)
bool signal, full_regs;
@@ -99,6 +101,27 @@ void unwind_module_init(struct module *mod, void *orc_ip, size_t orc_ip_size,
void *orc, size_t orc_size) {}
#endif

+static inline
+unsigned long unwind_recover_kretprobe(struct unwind_state *state,
+ unsigned long addr, unsigned long *addr_p)
+{
+ return is_kretprobe_trampoline(addr) ?
+ kretprobe_find_ret_addr(state->task, addr_p, &state->kr_cur) :
+ addr;
+}
+
+/* Recover the return address modified by kretprobe and ftrace_graph. */
+static inline
+unsigned long unwind_recover_ret_addr(struct unwind_state *state,
+ unsigned long addr, unsigned long *addr_p)
+{
+ unsigned long ret;
+
+ ret = ftrace_graph_ret_addr(state->task, &state->graph_idx,
+ addr, addr_p);
+ return unwind_recover_kretprobe(state, ret, addr_p);
+}
+
/*
* This disables KASAN checking when reading a value from another task's stack,
* since the other task could be running on another CPU and could have poisoned
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index d7c44b257f7f..8e1c50c86e5d 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -240,8 +240,7 @@ static bool update_stack_state(struct unwind_state *state,
else {
addr_p = unwind_get_return_address_ptr(state);
addr = READ_ONCE_TASK_STACK(state->task, *addr_p);
- state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
- addr, addr_p);
+ state->ip = unwind_recover_ret_addr(state, addr, addr_p);
}

/* Save the original stack pointer for unwind_dump(): */
diff --git a/arch/x86/kernel/unwind_guess.c b/arch/x86/kernel/unwind_guess.c
index c49f10ffd8cd..884d68a6e714 100644
--- a/arch/x86/kernel/unwind_guess.c
+++ b/arch/x86/kernel/unwind_guess.c
@@ -15,8 +15,7 @@ unsigned long unwind_get_return_address(struct unwind_state *state)

addr = READ_ONCE_NOCHECK(*state->sp);

- return ftrace_graph_ret_addr(state->task, &state->graph_idx,
- addr, state->sp);
+ return unwind_recover_ret_addr(state, addr, state->sp);
}
EXPORT_SYMBOL_GPL(unwind_get_return_address);

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index a1202536fc57..e6f7592790af 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -534,9 +534,8 @@ bool unwind_next_frame(struct unwind_state *state)
if (!deref_stack_reg(state, ip_p, &state->ip))
goto err;

- state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
- state->ip, (void *)ip_p);
-
+ state->ip = unwind_recover_ret_addr(state, state->ip,
+ (unsigned long *)ip_p);
state->sp = sp;
state->regs = NULL;
state->prev_regs = NULL;
@@ -549,7 +548,18 @@ bool unwind_next_frame(struct unwind_state *state)
(void *)orig_ip);
goto err;
}
-
+ /*
+ * There is a small chance to interrupt at the entry of
+ * __kretprobe_trampoline() where the ORC info doesn't exist.
+ * That point is right after the RET to __kretprobe_trampoline()
+ * which was modified return address.
+ * At that point, the @addr_p of the unwind_recover_kretprobe()
+ * (this has to point the address of the stack entry storing
+ * the modified return address) must be "SP - (a stack entry)"
+ * because SP is incremented by the RET.
+ */
+ state->ip = unwind_recover_kretprobe(state, state->ip,
+ (unsigned long *)(state->sp - sizeof(long)));
state->regs = (struct pt_regs *)sp;
state->prev_regs = NULL;
state->full_regs = true;
@@ -562,6 +572,9 @@ bool unwind_next_frame(struct unwind_state *state)
(void *)orig_ip);
goto err;
}
+ /* See UNWIND_HINT_TYPE_REGS case comment. */
+ state->ip = unwind_recover_kretprobe(state, state->ip,
+ (unsigned long *)(state->sp - sizeof(long)));

if (state->full_regs)
state->prev_regs = state->regs;