Messages in this thread Patch in this message | | | From | Kees Cook <> | Subject | [PATCH v2 1/3] fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL | Date | Fri, 2 Sep 2022 13:43:49 -0700 |
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With CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y enabled, we observe a runtime panic while running Android's Compatibility Test Suite's (CTS) android.hardware.input.cts.tests. This is stemming from a strlen() call in hidinput_allocate().
__compiletime_strlen() is implemented in terms of __builtin_object_size(), then does an array access to check for NUL-termination. A quirk of __builtin_object_size() is that for strings whose values are runtime dependent, __builtin_object_size(str, 1 or 0) returns the maximum size of possible values when those sizes are determinable at compile time. Example:
static const char *v = "FOO BAR"; static const char *y = "FOO BA"; unsigned long x (int z) { // Returns 8, which is: // max(__builtin_object_size(v, 1), __builtin_object_size(y, 1)) return __builtin_object_size(z ? v : y, 1); }
So when FORTIFY_SOURCE is enabled, the current implementation of __compiletime_strlen() will try to access beyond the end of y at runtime using the size of v. Mixed with UBSAN_LOCAL_BOUNDS we get a fault.
hidinput_allocate() has a local C string whose value is control flow dependent on a switch statement, so __builtin_object_size(str, 1) evaluates to the maximum string length, making all other cases fault on the last character check. hidinput_allocate() could be cleaned up to avoid runtime calls to strlen() since the local variable can only have literal values, so there's no benefit to trying to fortify the strlen call site there.
Perform a __builtin_constant_p() check against index 0 earlier in the macro to filter out the control-flow-dependant case. Add a KUnit test for checking the expected behavioral characteristics of FORTIFY_SOURCE internals.
Cc: Nathan Chancellor <nathan@kernel.org> Cc: Tom Rix <trix@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: "Steven Rostedt (Google)" <rostedt@goodmis.org> Cc: David Gow <davidgow@google.com> Cc: Yury Norov <yury.norov@gmail.com> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Sander Vanheule <sander@svanheule.net> Cc: linux-hardening@vger.kernel.org Cc: llvm@lists.linux.dev Co-developed-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/fortify-string.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index eed2119b23c5..07d5d1921eff 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -19,7 +19,8 @@ void __write_overflow_field(size_t avail, size_t wanted) __compiletime_warning(" unsigned char *__p = (unsigned char *)(p); \ size_t __ret = (size_t)-1; \ size_t __p_size = __builtin_object_size(p, 1); \ - if (__p_size != (size_t)-1) { \ + if (__p_size != (size_t)-1 && \ + __builtin_constant_p(*__p)) { \ size_t __p_len = __p_size - 1; \ if (__builtin_constant_p(__p[__p_len]) && \ __p[__p_len] == '\0') \ -- 2.34.1
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