From: Kees Cook keescook@chromium.org
[ Upstream commit d07c0acb4f41cc42a0d97530946965b3e4fa68c1 ]
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 Reviewed-by: Nick Desaulniers ndesaulniers@google.com Tested-by: Android Treehugger Robot Link: https://android-review.googlesource.com/c/kernel/common/+/2206839 Co-developed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Kees Cook keescook@chromium.org Signed-off-by: Sasha Levin sashal@kernel.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 3b401fa0f374..fce2fb2fc962 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') \