objtool: Add UACCESS validation

It is important that UACCESS regions are as small as possible;
furthermore the UACCESS state is not scheduled, so doing anything that
might directly call into the scheduler will cause random code to be
ran with UACCESS enabled.

Teach objtool too track UACCESS state and warn about any CALL made
while UACCESS is enabled. This very much includes the __fentry__()
and __preempt_schedule() calls.

Note that exceptions _do_ save/restore the UACCESS state, and therefore
they can drive preemption. This also means that all exception handlers
must have an otherwise redundant UACCESS disable instruction;
therefore ignore this warning for !STT_FUNC code (exception handlers
are not normal functions).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/tools/objtool/special.c b/tools/objtool/special.c
index 50af4e1274b3..4e50563d87c6 100644
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "builtin.h"
 #include "special.h"
 #include "warn.h"
 
@@ -42,6 +43,7 @@
 #define ALT_NEW_LEN_OFFSET	11
 
 #define X86_FEATURE_POPCNT (4*32+23)
+#define X86_FEATURE_SMAP   (9*32+20)
 
 struct special_entry {
 	const char *sec;
@@ -110,6 +112,22 @@ static int get_alt_entry(struct elf *elf, struct special_entry *entry,
 		 */
 		if (feature == X86_FEATURE_POPCNT)
 			alt->skip_orig = true;
+
+		/*
+		 * If UACCESS validation is enabled; force that alternative;
+		 * otherwise force it the other way.
+		 *
+		 * What we want to avoid is having both the original and the
+		 * alternative code flow at the same time, in that case we can
+		 * find paths that see the STAC but take the NOP instead of
+		 * CLAC and the other way around.
+		 */
+		if (feature == X86_FEATURE_SMAP) {
+			if (uaccess)
+				alt->skip_orig = true;
+			else
+				alt->skip_alt = true;
+		}
 	}
 
 	orig_rela = find_rela_by_dest(sec, offset + entry->orig);