[Devel] [PATCH RH7] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK

[Pavel Tikhomirov]

From: "Masami Hiramatsu (Google)" <mhiramat at kernel.org>

Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for stopping
speculative execution after RET instruction, kprobes always failes to
check the probed instruction boundary by decoding the function body if
the probed address is after such sequence. (Note that some conditional
code blocks will be placed after function return, if compiler decides
it is not on the hot path.)

This is because kprobes expects kgdb puts the INT3 as a software
breakpoint and it will replace the original instruction.
But these INT3 are not such purpose, it doesn't need to recover the
original instruction.

To avoid this issue, kprobes checks whether the INT3 is owned by
kgdb or not, and if so, stop decoding and make it fail. The other
INT3 will come from CONFIG_RETHUNK/CONFIG_SLS and those can be
treated as a one-byte instruction.

Fixes: e463a09af2f0 ("x86: Add straight-line-speculation mitigation")
Suggested-by: Peter Zijlstra <peterz at infradead.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat at kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: stable at vger.kernel.org
Link: https://lore.kernel.org/r/167146051026.1374301.392728975473572291.stgit@devnote3

Changes:
- s/INT3_INSN_OPCODE/BREAKPOINT_INSTRUCTION/
- include kgdb_has_hit_break implementation

If function return (int3 instruction) got compiled in assembly code into
the middle of the function, then we failed to insert kprobe at any point
after that. To be able to insert kprobes everythere in the function code
we need this patch.

(cherry picked from commit 1993bf97992df2d560287f3c4120eda57426843d)
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 arch/x86/kernel/kprobes/core.c | 10 +++++++---
 include/linux/kgdb.h           |  1 +
 kernel/debug/debug_core.c      | 12 ++++++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 2be0dde130f14..9dc7f48e84ea9 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -48,6 +48,7 @@
 #include <linux/module.h>
 #include <linux/kdebug.h>
 #include <linux/kallsyms.h>
+#include <linux/kgdb.h>
 #include <linux/ftrace.h>
 #include <linux/frame.h>
 #include <linux/kasan.h>
@@ -300,12 +301,15 @@ static int __kprobes can_probe(unsigned long paddr)
 		kernel_insn_init(&insn, (void *)__addr, MAX_INSN_SIZE);
 		insn_get_length(&insn);
 
+#ifdef CONFIG_KGDB
 		/*
-		 * Another debugging subsystem might insert this breakpoint.
-		 * In that case, we can't recover it.
+		 * If there is a dynamically installed kgdb sw breakpoint,
+		 * this function should not be probed.
 		 */
-		if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
+		if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION &&
+		    kgdb_has_hit_break(addr))
 			return 0;
+#endif
 		addr += insn.length;
 	}
 
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index 6b06d378f3dfe..47a8fd917b3b5 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -305,6 +305,7 @@ extern int kgdb_hex2mem(char *buf, char *mem, int count);
 
 extern int kgdb_isremovedbreak(unsigned long addr);
 extern void kgdb_schedule_breakpoint(void);
+extern int kgdb_has_hit_break(unsigned long addr);
 
 extern int
 kgdb_handle_exception(int ex_vector, int signo, int err_code,
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 8865caec45fbc..e5872e38fae51 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -356,6 +356,18 @@ int kgdb_isremovedbreak(unsigned long addr)
 	return 0;
 }
 
+int kgdb_has_hit_break(unsigned long addr)
+{
+	int i;
+
+	for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
+		if (kgdb_break[i].state == BP_ACTIVE &&
+		    kgdb_break[i].bpt_addr == addr)
+			return 1;
+	}
+	return 0;
+}
+
 int dbg_remove_all_break(void)
 {
 	int error;
-- 
2.43.0