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 <pet...@infradead.org>
Signed-off-by: Daniel Thompson <daniel.thomp...@linaro.org>
---
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
