[RFC PATCH 1/4] kgdb: Honour the kprobe blacklist when setting breakpoints

[Daniel Thompson]

Currently kgdb has absolutely no safety rails in place to discourage or
prevent a user from placing a breakpoint in dangerous places such as
the debugger's own trap entry/exit and other places where it is not safe
to take synchronous traps.

Modify the default implementation of kgdb_validate_break_address() so
that we honour the kprobe blacklist (if there is one). The resulting
blacklist will include code that kgdb could, in fact, debug but I think
we can assume that anyone with sufficient knowledge to meaningfully
debug that code would trivially be able to find and remove the safety
rail if they need to.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---
 kernel/debug/debug_core.c | 11 +++++++++++
 kernel/debug/kdb/kdb_bp.c |  9 +++++++++
 2 files changed, 20 insertions(+)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index ef94e906f05a..81f56d616e04 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -56,6 +56,7 @@
 #include <linux/vmacache.h>
 #include <linux/rcupdate.h>
 #include <linux/irq.h>
+#include <linux/kprobes.h>
 
 #include <asm/cacheflush.h>
 #include <asm/byteorder.h>
@@ -188,6 +189,16 @@ int __weak kgdb_validate_break_address(unsigned long addr)
 {
 	struct kgdb_bkpt tmp;
 	int err;
+
+	/*
+	 * Disallow breakpoints that are marked as unsuitable for kprobing.
+	 * This check is a little over-zealous because it does include
+	 * code that kgdb is entirely capable of debugging but in exchange
+	 * we can avoid recursive trapping (and all the problems that brings).
+	 */
+	if (within_kprobe_blacklist(addr))
+		return -EINVAL;
+
 	/* Validate setting the breakpoint and then removing it.  If the
 	 * remove fails, the kernel needs to emit a bad message because we
 	 * are deep trouble not being able to put things back the way we
diff --git a/kernel/debug/kdb/kdb_bp.c b/kernel/debug/kdb/kdb_bp.c
index d7ebb2c79cb8..ec4940146612 100644
--- a/kernel/debug/kdb/kdb_bp.c
+++ b/kernel/debug/kdb/kdb_bp.c
@@ -306,6 +306,15 @@ static int kdb_bp(int argc, const char **argv)
 	if (!template.bp_addr)
 		return KDB_BADINT;
 
+	/*
+	 * This check is redundant (since the breakpoint machinery should
+	 * be doing the same check during kdb_bp_install) but gives the
+	 * user immediate feedback.
+	 */
+	diag = kgdb_validate_break_address(template.bp_addr);
+	if (diag)
+		return diag;
+
 	/*
 	 * Find an empty bp structure to allocate
 	 */
-- 
2.25.4