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objtool: Documentation: document UACCESS warnings 

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>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Nick Desaulniers 2020-03-26 11:37:06 -07:00 committed by Ingo Molnar
parent 6f8ca67683
commit e378fa17d3
1 changed files with 26 additions and 0 deletions
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26
tools/objtool/Documentation/stack-validation.txt
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@ -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.