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Subject[tip:x86/paravirt] x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()
Commit-ID:  11e86dc7f2746210f9c7dc10deaa7658f8dc8350
Gitweb: https://git.kernel.org/tip/11e86dc7f2746210f9c7dc10deaa7658f8dc8350
Author: Ingo Molnar <mingo@kernel.org>
AuthorDate: Thu, 25 Apr 2019 11:50:39 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 25 Apr 2019 12:00:44 +0200

x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()

paravirt_patch_call() currently handles patching failures inconsistently:
we generate a warning in the retpoline case, but don't in other cases where
we might end up with a non-working kernel as well.

So just convert it all to a BUG_ON(), these patching calls are *not* supposed
to fail, and if they do we want to know it immediately.

This also makes the kernel smaller and removes an #ifdef ugly.

I tried it with a richly paravirt-enabled kernel and no patching bugs
were detected.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20190425095039.GC115378@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/paravirt.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 7f9121f2fdac..544d386ded45 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -73,21 +73,21 @@ struct branch {
static unsigned paravirt_patch_call(void *insnbuf, const void *target,
unsigned long addr, unsigned len)
{
+ const int call_len = 5;
struct branch *b = insnbuf;
- unsigned long delta = (unsigned long)target - (addr+5);
+ unsigned long delta = (unsigned long)target - (addr+call_len);

- if (len < 5) {
-#ifdef CONFIG_RETPOLINE
- WARN_ONCE(1, "Failing to patch indirect CALL in %ps\n", (void *)addr);
-#endif
- return len; /* call too long for patch site */
+ if (len < call_len) {
+ pr_warn("paravirt: Failed to patch indirect CALL at %ps\n", (void *)addr);
+ /* Kernel might not be viable if patching fails, bail out: */
+ BUG_ON(1);
}

b->opcode = 0xe8; /* call */
b->delta = delta;
- BUILD_BUG_ON(sizeof(*b) != 5);
+ BUILD_BUG_ON(sizeof(*b) != call_len);

- return 5;
+ return call_len;
}

#ifdef CONFIG_PARAVIRT_XXL
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