On Fri, Aug 30, 2019 at 1:38 AM Josh Poimboeuf [off-list ref] wrote:

On Thu, Aug 29, 2019 at 10:24:45AM +0200, Peter Zijlstra wrote:

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On Wed, Aug 28, 2019 at 03:01:34PM -0500, Josh Poimboeuf wrote:

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On Wed, Aug 28, 2019 at 10:56:25AM -0700, Randy Dunlap wrote:

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drivers/xen/gntdev.o: warning: objtool: gntdev_copy()+0x229: call to __ubsan_handle_out_of_bounds() with UACCESS enabled

Easy one :-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0c8e17f946cd..6a935ab93149 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c

@@ -483,6 +483,7 @@ static const char *uaccess_safe_builtin[] = {
        "ubsan_type_mismatch_common",
        "__ubsan_handle_type_mismatch",
        "__ubsan_handle_type_mismatch_v1",
+       "__ubsan_handle_out_of_bounds",
        /* misc */
        "csum_partial_copy_generic",
        "__memcpy_mcsafe",

then I get this one:

lib/ubsan.o: warning: objtool: __ubsan_handle_out_of_bounds()+0x5d: call to ubsan_prologue() with UACCESS enabled

And of course I jinxed it by calling it easy.

Peter, how do you want to handle this?

Should we just disable UACCESS checking in lib/ubsan.c?

No, that is actually unsafe and could break things (as would you patch
above).

Oops.  -EFIXINGTOOMANYOBJTOOLISSUESATONCE

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I'm thinking the below patch ought to cure things:

---
Subject: x86/uaccess: Don't leak the AC flags into __get_user() argument evalidation

s/evalidation/evaluation

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Identical to __put_user(); the __get_user() argument evalution will too
leak UBSAN crud into the __uaccess_begin() / __uaccess_end() region.
While uncommon this was observed to happen for:

  drivers/xen/gntdev.c: if (__get_user(old_status, batch->status[i]))

where UBSAN added array bound checking.

This complements commit:

  6ae865615fc4 ("x86/uaccess: Dont leak the AC flag into __put_user() argument evaluation")

Reported-by: Randy Dunlap <redacted>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: luto@kernel.org
---
 arch/x86/include/asm/uaccess.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 9c4435307ff8..35c225ede0e4 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h

@@ -444,8 +444,10 @@ __pu_label:                                                      \
 ({                                                                   \
      int __gu_err;                                                   \
      __inttype(*(ptr)) __gu_val;                                     \
+     __typeof__(ptr) __gu_ptr = (ptr);                               \
+     __typeof__(size) __gu_size = (size);                            \
      __uaccess_begin_nospec();                                       \
-     __get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);    \
+     __get_user_size(__gu_val, __gu_ptr, __gu_size, __gu_err, -EFAULT);      \
      __uaccess_end();                                                \
      (x) = (__force __typeof__(*(ptr)))__gu_val;                     \
      __builtin_expect(__gu_err, 0);                                  \

Reviewed-by: Josh Poimboeuf <redacted>

Tested-by Sedat Dilek [off-list ref]

- Sedat -