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Subject[PATCH v3 -tip] [BUGFIX] x86/kprobes: Fix to recover instructions on optimized path
Date
Current probed-instruction recovery expects that only breakpoint
instruction modifies instruction. However, since kprobes jump
optimization can replace original instructions with a jump,
that expectation is not enough. And it may cause instruction
decoding failure on the function where an optimized probe
already exists.

This bug can reproduce easily as below.

1) find a target function address (any kprobe-able function is OK)
$ grep __secure_computing /proc/kallsyms
ffffffff810c19d0 T __secure_computing
2) decode the function
$ objdump -d vmlinux --start-address=0xffffffff810c19d0 --stop-address=0xffffffff810c19eb
vmlinux: file format elf64-x86-64


Disassembly of section .text:

ffffffff810c19d0 <__secure_computing>:
ffffffff810c19d0: 55 push %rbp
ffffffff810c19d1: 48 89 e5 mov %rsp,%rbp
ffffffff810c19d4: e8 67 8f 72 00 callq ffffffff817ea940 <mcount>
ffffffff810c19d9: 65 48 8b 04 25 40 b8 mov %gs:0xb840,%rax
ffffffff810c19e0: 00 00
ffffffff810c19e2: 83 b8 88 05 00 00 01 cmpl $0x1,0x588(%rax)
ffffffff810c19e9: 74 05 je ffffffff810c19f0 <__secure_computing+0x20>
3) put a kprobe-event at an optimize-able place, where no
call/jump places within the 5 bytes.
$ su -
# cd /sys/kernel/debug/tracing
# echo p __secure_computing+0x9 > kprobe_events
4) enable it and check it is optimized.
# echo 1 > events/kprobes/p___secure_computing_9/enable
# cat ../kprobes/list
ffffffff810c19d9 k __secure_computing+0x9 [OPTIMIZED]
5) put another kprobe on an instruction after previous probe in
the same function.
# echo p __secure_computing+0x12 >> kprobe_events
bash: echo: write error: Invalid argument
# dmesg | tail -n 1
[ 1666.500016] Probing address(0xffffffff810c19e2) is not an instruction boundary.
6) however, if the kprobes optimization is disabled, it works.
# echo 0 > /proc/sys/debug/kprobes-optimization
# cat ../kprobes/list
ffffffff810c19d9 k __secure_computing+0x9
# echo p __secure_computing+0x12 >> kprobe_events
(no error)
This is because kprobes doesn't recover the instruction
which is overwritten with a relative jump by another kprobe
when finding instruction boundary.
It only recovers the breakpoint instruction.

This patch fixes kprobes to recover such instructions.

With this fix:

# echo p __secure_computing+0x9 > kprobe_events
# echo 1 > events/kprobes/p___secure_computing_9/enable
# cat ../kprobes/list
ffffffff810c1aa9 k __secure_computing+0x9 [OPTIMIZED]
# echo p __secure_computing+0x12 >> kprobe_events
# cat ../kprobes/list
ffffffff810c1aa9 k __secure_computing+0x9 [OPTIMIZED]
ffffffff810c1ab2 k __secure_computing+0x12 [DISABLED]
Changes in v3:
- Fix a build error when CONFIG_OPTPROBE=n. (Thanks, Ingo!)
To fix the error, split optprobe instruction recovering
path from kprobes path.
- Cleanup comments/styles.

Changes in v2:
- Fix a bug to recover original instruction address in
RIP-relative instruction fixup.
- Moved on tip/master.


Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
---
arch/x86/kernel/kprobes.c | 128 ++++++++++++++++++++++++++++-----------------
include/linux/kprobes.h | 6 ++
kernel/kprobes.c | 2 -
3 files changed, 88 insertions(+), 48 deletions(-)
diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index 7da647d..77ea425 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -207,14 +207,9 @@ retry:
}
}

-/* Recover the probed instruction at addr for further analysis. */
-static int recover_probed_instruction(kprobe_opcode_t *buf, unsigned long addr)
+static unsigned long __recover_probed_insn(struct kprobe *kp,
+ kprobe_opcode_t *buf)
{
- struct kprobe *kp;
- kp = get_kprobe((void *)addr);
- if (!kp)
- return -EINVAL;
-
/*
* Basically, kp->ainsn.insn has an original instruction.
* However, RIP-relative instruction can not do single-stepping
@@ -230,14 +225,63 @@ static int recover_probed_instruction(kprobe_opcode_t *buf, unsigned long addr)
*/
memcpy(buf, kp->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
buf[0] = kp->opcode;
- return 0;
+ return (unsigned long)buf;
+}
+
+#ifdef CONFIG_OPTPROBES
+static unsigned long __recover_optprobed_insn(struct kprobe *kp,
+ kprobe_opcode_t *buf,
+ unsigned long addr)
+{
+ long offs = addr - (unsigned long)kp->addr - 1;
+ struct optimized_kprobe *op = container_of(kp, struct optimized_kprobe, kp);
+
+ /*
+ * If the kprobe can be optimized, original bytes which can be
+ * overwritten by jump destination address. In this case, original
+ * bytes must be recovered from op->optinsn.copied_insn buffer.
+ */
+ memcpy(buf, (void *)addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
+ if (addr == (unsigned long)kp->addr) {
+ buf[0] = kp->opcode;
+ memcpy(buf + 1, op->optinsn.copied_insn, RELATIVE_ADDR_SIZE);
+ } else
+ memcpy(buf, op->optinsn.copied_insn + offs, RELATIVE_ADDR_SIZE - offs);
+
+ return (unsigned long)buf;
+}
+#endif
+
+/*
+ * Recover the probed instruction at addr for further analysis.
+ * Caller must lock kprobes by kprobe_mutex, or disable preemption
+ * for preventing to release referencing kprobes.
+ */
+static unsigned long recover_probed_instruction(kprobe_opcode_t *buf,
+ unsigned long addr)
+{
+ struct kprobe *kp;
+#ifdef CONFIG_OPTPROBES
+ int i;
+
+ for (i = 0; i < RELATIVEJUMP_SIZE; i++) {
+ kp = get_kprobe((void *)addr - i);
+ if (kp && kprobe_optready(kp))
+ return __recover_optprobed_insn(kp, buf, addr);
+ }
+#endif
+ kp = get_kprobe((void *)addr);
+ /* There is no probe, return original address */
+ if (!kp)
+ return addr;
+
+ return __recover_probed_insn(kp, buf);
}

/* Check if paddr is at an instruction boundary */
static int __kprobes can_probe(unsigned long paddr)
{
- int ret;
- unsigned long addr, offset = 0;
+ unsigned long addr, __addr, offset = 0;
struct insn insn;
kprobe_opcode_t buf[MAX_INSN_SIZE];

@@ -247,26 +291,24 @@ static int __kprobes can_probe(unsigned long paddr)
/* Decode instructions */
addr = paddr - offset;
while (addr < paddr) {
- kernel_insn_init(&insn, (void *)addr);
- insn_get_opcode(&insn);
-
/*
* Check if the instruction has been modified by another
* kprobe, in which case we replace the breakpoint by the
* original instruction in our buffer.
+ * Also, jump optimization will change the breakpoint to
+ * relative-jump. Since the relative-jump itself is
+ * normally used, we just go through if there is no kprobe.
*/
- if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
- ret = recover_probed_instruction(buf, addr);
- if (ret)
- /*
- * Another debugging subsystem might insert
- * this breakpoint. In that case, we can't
- * recover it.
- */
- return 0;
- kernel_insn_init(&insn, buf);
- }
+ __addr = recover_probed_instruction(buf, addr);
+ kernel_insn_init(&insn, (void *)__addr);
insn_get_length(&insn);
+
+ /*
+ * Another debugging subsystem might insert this breakpoint.
+ * In that case, we can't recover it.
+ */
+ if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
+ return 0;
addr += insn.length;
}

@@ -302,21 +344,17 @@ static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
static int __kprobes __copy_instruction(u8 *dest, u8 *src, int recover)
{
struct insn insn;
- int ret;
kprobe_opcode_t buf[MAX_INSN_SIZE];
+ u8 *orig_src = src; /* Back up original src for RIP calculation */
+
+ if (recover)
+ src = (u8 *)recover_probed_instruction(buf, (unsigned long)src);

kernel_insn_init(&insn, src);
- if (recover) {
- insn_get_opcode(&insn);
- if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
- ret = recover_probed_instruction(buf,
- (unsigned long)src);
- if (ret)
- return 0;
- kernel_insn_init(&insn, buf);
- }
- }
insn_get_length(&insn);
+ /* Another subsystem puts a breakpoint, failed to recover */
+ if (recover && insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
+ return 0;
memcpy(dest, insn.kaddr, insn.length);

#ifdef CONFIG_X86_64
@@ -337,8 +375,7 @@ static int __kprobes __copy_instruction(u8 *dest, u8 *src, int recover)
* extension of the original signed 32-bit displacement would
* have given.
*/
- newdisp = (u8 *) src + (s64) insn.displacement.value -
- (u8 *) dest;
+ newdisp = (u8 *) orig_src + (s64) insn.displacement.value - (u8 *) dest;
BUG_ON((s64) (s32) newdisp != newdisp); /* Sanity check. */
disp = (u8 *) dest + insn_offset_displacement(&insn);
*(s32 *) disp = (s32) newdisp;
@@ -1271,8 +1308,7 @@ static int insn_jump_into_range(struct insn *insn, unsigned long start, int len)
/* Decode whole function to ensure any instructions don't jump into target */
static int __kprobes can_optimize(unsigned long paddr)
{
- int ret;
- unsigned long addr, size = 0, offset = 0;
+ unsigned long addr, __addr, size = 0, offset = 0;
struct insn insn;
kprobe_opcode_t buf[MAX_INSN_SIZE];

@@ -1301,15 +1337,12 @@ static int __kprobes can_optimize(unsigned long paddr)
* we can't optimize kprobe in this function.
*/
return 0;
- kernel_insn_init(&insn, (void *)addr);
- insn_get_opcode(&insn);
- if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
- ret = recover_probed_instruction(buf, addr);
- if (ret)
- return 0;
- kernel_insn_init(&insn, buf);
- }
+ __addr = recover_probed_instruction(buf, addr);
+ kernel_insn_init(&insn, (void *)__addr);
insn_get_length(&insn);
+ /* Another subsystem puts a breakpoint */
+ if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
+ return 0;
/* Recover address */
insn.kaddr = (void *)addr;
insn.next_byte = (void *)(addr + insn.length);
@@ -1366,6 +1399,7 @@ void __kprobes arch_remove_optimized_kprobe(struct optimized_kprobe *op)
/*
* Copy replacing target instructions
* Target instructions MUST be relocatable (checked inside)
+ * This is called when new aggr(opt)probe is allocated or reused.
*/
int __kprobes arch_prepare_optimized_kprobe(struct optimized_kprobe *op)
{
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index dce6e4d..6abec49 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -293,6 +293,12 @@ extern int proc_kprobes_optimization_handler(struct ctl_table *table,
size_t *length, loff_t *ppos);
#endif

+extern int kprobe_optready(struct kprobe *p);
+#else /* CONFIG_OPTPROBES */
+static inline int kprobe_optready(struct kprobe *p)
+{
+ return 0;
+}
#endif /* CONFIG_OPTPROBES */

/* Get the kprobe at this addr (if any) - called with preemption disabled */
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 9788c0e..c52c68b 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -403,7 +403,7 @@ static __kprobes void free_aggr_kprobe(struct kprobe *p)
}

/* Return true(!0) if the kprobe is ready for optimization. */
-static inline int kprobe_optready(struct kprobe *p)
+int kprobe_optready(struct kprobe *p)
{
struct optimized_kprobe *op;



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