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Subject[PATCH v2] objtool: Documentation: document UACCESS warnings
From
Compiling with Clang and CONFIG_KASAN=y was exposing a few warnings:
call to memset() with UACCESS enabled

Document how to fix these for future travelers.

Link: https://github.com/ClangBuiltLinux/linux/issues/876
Suggested-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Suggested-by: Matt Helsley <mhelsley@vmware.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V1 -> V2:
* fix typo of listing uaccess_enable() twice rather than
uaccess_disable() as per Matt and Kamalesh.
* fix type of "should called" to "should be called" as per Randy.
* Mention non-obvious compiler instrumentation ie. -pg/mcount
-mfentry/fentry via tracing as per Peter.
* Add sentence "It also helps verify..."
* Add potential fix "1) remove explicit..."


.../Documentation/stack-validation.txt | 26 +++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index de094670050b..faa47c3aafae 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/tools/objtool/Documentation/stack-validation.txt
@@ -289,6 +289,32 @@ they mean, and suggestions for how to fix them.
might be corrupt due to a gcc bug. For more details, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646

+9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled
+
+ This means that an unexpected call to a non-whitelisted function exists
+ outside of arch-specific guards.
+ X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end()
+ ARM: PAN: uaccess_enable()/uaccess_disable()
+
+ These functions should be called to denote a minimal critical section around
+ access to __user variables. See also: https://lwn.net/Articles/517475/
+
+ The intention of the warning is to prevent calls to funcB() from eventually
+ calling schedule(), potentially leaking the AC flags state, and not
+ restoring them correctly.
+
+ It also helps verify that there are no unexpected calls to funcB() which may
+ access user space pages with protections against doing so disabled.
+
+ To fix, either:
+ 1) remove explicit calls to funcB() from funcA().
+ 2) add the correct guards before and after calls to low level functions like
+ __get_user_size()/__put_user_size().
+ 3) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
+ funcB obviously does not call schedule(), and is marked notrace (since
+ function tracing inserts additional calls, which is not obvious from the
+ sources).
+

If the error doesn't seem to make sense, it could be a bug in objtool.
Feel free to ask the objtool maintainer for help.
--
2.26.0.rc2.310.g2932bb562d-goog
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