[PATCH v3 19/39] x86/ibt,kprobes: Cure sym+0 equals fentry woes

Peter Zijlstra
2022-03-03T11:33:01Z

Rework _kprobe_addr() to only ever do 2 kallsyms calls and require
only a single architecture/weak function -- although currently powerpc
still uses 2.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/powerpc/kernel/kprobes.c | 34 +++++++++++++--------
arch/x86/kernel/kprobes/core.c | 28 ++++++++++++++---
include/linux/kprobes.h | 3 +
kernel/kprobes.c | 66 ++++++++++++++++++++++++++++++++---------
4 files changed, 98 insertions(+), 33 deletions(-)

--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -105,6 +105,27 @@ kprobe_opcode_t *kprobe_lookup_name(cons
return addr;
}

+static bool arch_kprobe_on_func_entry(unsigned long offset)
+{
+#ifdef PPC64_ELF_ABI_v2
+#ifdef CONFIG_KPROBES_ON_FTRACE
+ return offset <= 16;
+#else
+ return offset <= 8;
+#endif
+#else
+ return !offset;
+#endif
+}
+
+/* XXX try and fold the magic of kprobe_lookup_name() in this */
+kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
+ bool *on_func_entry)
+{
+ *on_func_entry = arch_kprobe_on_func_entry(offset);
+ return (kprobe_opcode_t *)(addr + offset);
+}
+
void *alloc_insn_page(void)
{
void *page;
@@ -218,19 +239,6 @@ static nokprobe_inline void set_current_
kcb->kprobe_saved_msr = regs->msr;
}

-bool arch_kprobe_on_func_entry(unsigned long offset)
-{
-#ifdef PPC64_ELF_ABI_v2
-#ifdef CONFIG_KPROBES_ON_FTRACE
- return offset <= 16;
-#else
- return offset <= 8;
-#endif
-#else
- return !offset;
-#endif
-}
-
void arch_prepare_kretprobe(struct kretprobe_instance *ri, struct pt_regs *regs)
{
ri->ret_addr = (kprobe_opcode_t *)regs->link;
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -52,6 +52,7 @@
#include <asm/insn.h>
#include <asm/debugreg.h>
#include <asm/set_memory.h>
+#include <asm/ibt.h>

#include "common.h"

@@ -197,13 +198,14 @@ __recover_probed_insn(kprobe_opcode_t *b

kp = get_kprobe((void *)addr);
faddr = ftrace_location(addr);
+
/*
- * Addresses inside the ftrace location are refused by
- * arch_check_ftrace_location(). Something went terribly wrong
- * if such an address is checked here.
+ * In case addr maps to sym+0 ftrace_location() might return something
+ * other than addr. In that case consider it the same as !faddr.
*/
- if (WARN_ON(faddr && faddr != addr))
- return 0UL;
+ if (faddr && faddr != addr)
+ faddr = 0;
+
/*
* Use the current code if it is not modified by Kprobe
* and it cannot be modified by ftrace.
@@ -301,6 +303,22 @@ static int can_probe(unsigned long paddr
return (addr == paddr);
}

+/* If the x86 support IBT (ENDBR) it must be skipped. */
+kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
+ bool *on_func_entry)
+{
+ if (is_endbr(*(u32 *)addr)) {
+ *on_func_entry = !offset || offset == 4;
+ if (*on_func_entry)
+ offset = 4;
+
+ } else {
+ *on_func_entry = !offset;
+ }
+
+ return (kprobe_opcode_t *)(addr + offset);
+}
+
/*
* Copy an instruction with recovering modified instruction by kprobes
* and adjust the displacement if the instruction uses the %rip-relative
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -265,7 +265,6 @@ extern int arch_init_kprobes(void);
extern void kprobes_inc_nmissed_count(struct kprobe *p);
extern bool arch_within_kprobe_blacklist(unsigned long addr);
extern int arch_populate_kprobe_blacklist(void);
-extern bool arch_kprobe_on_func_entry(unsigned long offset);
extern int kprobe_on_func_entry(kprobe_opcode_t *addr, const char *sym, unsigned long offset);

extern bool within_kprobe_blacklist(unsigned long addr);
@@ -384,6 +383,8 @@ static inline struct kprobe_ctlblk *get_
}

kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset);
+kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset, bool *on_func_entry);
+
int register_kprobe(struct kprobe *p);
void unregister_kprobe(struct kprobe *p);
int register_kprobes(struct kprobe **kps, int num);
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1489,24 +1489,68 @@ bool within_kprobe_blacklist(unsigned lo
}

/*
+ * arch_adjust_kprobe_addr - adjust the address
+ * @addr: symbol base address
+ * @offset: offset within the symbol
+ * @on_func_entry: was this @addr+@offset on the function entry
+ *
+ * Typically returns @addr + @offset, except for special cases where the
+ * function might be prefixed by a CFI landing pad, in that case any offset
+ * inside the landing pad is mapped to the first 'real' instruction of the
+ * symbol.
+ *
+ * Specifically, for things like IBT/BTI, skip the resp. ENDBR/BTI.C
+ * instruction at +0.
+ */
+kprobe_opcode_t *__weak arch_adjust_kprobe_addr(unsigned long addr,
+ unsigned long offset,
+ bool *on_func_entry)
+{
+ *on_func_entry = !offset;
+ return (kprobe_opcode_t *)(addr + offset);
+}
+
+/*
* If 'symbol_name' is specified, look it up and add the 'offset'
* to it. This way, we can specify a relative address to a symbol.
* This returns encoded errors if it fails to look up symbol or invalid
* combination of parameters.
*/
-static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
- const char *symbol_name, unsigned int offset)
+static kprobe_opcode_t *
+_kprobe_addr(kprobe_opcode_t *addr, const char *symbol_name,
+ unsigned long offset, bool *on_func_entry)
{
if ((symbol_name && addr) || (!symbol_name && !addr))
goto invalid;

if (symbol_name) {
+ /*
+ * Input: @sym + @offset
+ * Output: @addr + @offset
+ *
+ * NOTE: kprobe_lookup_name() does *NOT* fold the offset
+ * argument into it's output!
+ */
addr = kprobe_lookup_name(symbol_name, offset);
if (!addr)
return ERR_PTR(-ENOENT);
}

- addr = (kprobe_opcode_t *)(((char *)addr) + offset);
+ /*
+ * So here we have @addr + @offset, displace it into a new
+ * @addr' + @offset' where @addr' is the symbol start address.
+ */
+ addr = (void *)addr + offset;
+ if (!kallsyms_lookup_size_offset((unsigned long)addr, NULL, &offset))
+ return ERR_PTR(-ENOENT);
+ addr = (void *)addr - offset;
+
+ /*
+ * Then ask the architecture to re-combine them, taking care of
+ * magical function entry details while telling us if this was indeed
+ * at the start of the function.
+ */
+ addr = arch_adjust_kprobe_addr((unsigned long)addr, offset, on_func_entry);
if (addr)
return addr;

@@ -1516,7 +1560,8 @@ static kprobe_opcode_t *_kprobe_addr(kpr

static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
{
- return _kprobe_addr(p->addr, p->symbol_name, p->offset);
+ bool on_func_entry;
+ return _kprobe_addr(p->addr, p->symbol_name, p->offset, &on_func_entry);
}

/*
@@ -2047,11 +2092,6 @@ static int pre_handler_kretprobe(struct
}
NOKPROBE_SYMBOL(pre_handler_kretprobe);

-bool __weak arch_kprobe_on_func_entry(unsigned long offset)
-{
- return !offset;
-}
-
/**
* kprobe_on_func_entry() -- check whether given address is function entry
* @addr: Target address
@@ -2067,15 +2107,13 @@ bool __weak arch_kprobe_on_func_entry(un
*/
int kprobe_on_func_entry(kprobe_opcode_t *addr, const char *sym, unsigned long offset)
{
- kprobe_opcode_t *kp_addr = _kprobe_addr(addr, sym, offset);
+ bool on_func_entry;
+ kprobe_opcode_t *kp_addr = _kprobe_addr(addr, sym, offset, &on_func_entry);

if (IS_ERR(kp_addr))
return PTR_ERR(kp_addr);

- if (!kallsyms_lookup_size_offset((unsigned long)kp_addr, NULL, &offset))
- return -ENOENT;
-
- if (!arch_kprobe_on_func_entry(offset))
+ if (!on_func_entry)
return -EINVAL;

return 0;