|author||Josh Poimboeuf <email@example.com>||2016-08-30 08:04:16 -0500|
|committer||Linus Torvalds <firstname.lastname@example.org>||2016-08-30 10:10:21 -0700|
mm/usercopy: get rid of CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
There are three usercopy warnings which are currently being silenced for gcc 4.6 and newer: 1) "copy_from_user() buffer size is too small" compile warning/error This is a static warning which happens when object size and copy size are both const, and copy size > object size. I didn't see any false positives for this one. So the function warning attribute seems to be working fine here. Note this scenario is always a bug and so I think it should be changed to *always* be an error, regardless of CONFIG_DEBUG_STRICT_USER_COPY_CHECKS. 2) "copy_from_user() buffer size is not provably correct" compile warning This is another static warning which happens when I enable __compiletime_object_size() for new compilers (and CONFIG_DEBUG_STRICT_USER_COPY_CHECKS). It happens when object size is const, but copy size is *not*. In this case there's no way to compare the two at build time, so it gives the warning. (Note the warning is a byproduct of the fact that gcc has no way of knowing whether the overflow function will be called, so the call isn't dead code and the warning attribute is activated.) So this warning seems to only indicate "this is an unusual pattern, maybe you should check it out" rather than "this is a bug". I get 102(!) of these warnings with allyesconfig and the __compiletime_object_size() gcc check removed. I don't know if there are any real bugs hiding in there, but from looking at a small sample, I didn't see any. According to Kees, it does sometimes find real bugs. But the false positive rate seems high. 3) "Buffer overflow detected" runtime warning This is a runtime warning where object size is const, and copy size > object size. All three warnings (both static and runtime) were completely disabled for gcc 4.6 with the following commit: 2fb0815c9ee6 ("gcc4: disable __compiletime_object_size for GCC 4.6+") That commit mistakenly assumed that the false positives were caused by a gcc bug in __compiletime_object_size(). But in fact, __compiletime_object_size() seems to be working fine. The false positives were instead triggered by #2 above. (Though I don't have an explanation for why the warnings supposedly only started showing up in gcc 4.6.) So remove warning #2 to get rid of all the false positives, and re-enable warnings #1 and #3 by reverting the above commit. Furthermore, since #1 is a real bug which is detected at compile time, upgrade it to always be an error. Having done all that, CONFIG_DEBUG_STRICT_USER_COPY_CHECKS is no longer needed. Signed-off-by: Josh Poimboeuf <email@example.com> Cc: Kees Cook <firstname.lastname@example.org> Cc: Thomas Gleixner <email@example.com> Cc: Ingo Molnar <firstname.lastname@example.org> Cc: "H . Peter Anvin" <email@example.com> Cc: Andy Lutomirski <firstname.lastname@example.org> Cc: Steven Rostedt <email@example.com> Cc: Brian Gerst <firstname.lastname@example.org> Cc: Peter Zijlstra <email@example.com> Cc: Frederic Weisbecker <firstname.lastname@example.org> Cc: Byungchul Park <email@example.com> Cc: Nilay Vaish <firstname.lastname@example.org> Signed-off-by: Linus Torvalds <email@example.com>
Diffstat (limited to 'lib')
3 files changed, 0 insertions, 28 deletions
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2307d7c89dac..2e2cca509231 100644
@@ -1686,24 +1686,6 @@ config LATENCYTOP
Enable this option if you want to use the LatencyTOP tool
to find out which userspace is blocking on what kernel operations.
- bool "Strict user copy size checks"
- depends on ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
- depends on DEBUG_KERNEL && !TRACE_BRANCH_PROFILING
- Enabling this option turns a certain set of sanity checks for user
- copy operations into compile time failures.
- The copy_from_user() etc checks are there to help test if there
- are sufficient security checks on the length argument of
- the copy operation, by having gcc prove that the argument is
- within bounds.
- If unsure, say N.
menu "Runtime Testing"
diff --git a/lib/Makefile b/lib/Makefile
index cfa68eb269e4..5dc77a8ec297 100644
@@ -24,7 +24,6 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
is_single_threaded.o plist.o decompress.o kobject_uevent.o \
earlycpio.o seq_buf.o nmi_backtrace.o nodemask.o
-obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
lib-$(CONFIG_MMU) += ioremap.o
lib-$(CONFIG_SMP) += cpumask.o
lib-$(CONFIG_HAS_DMA) += dma-noop.o
diff --git a/lib/usercopy.c b/lib/usercopy.c
deleted file mode 100644
@@ -1,9 +0,0 @@
- WARN(1, "Buffer overflow detected!\n");