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SubjectRe: [PATCH] objtool: Documentation: document UACCESS warnings
On Mon, Mar 23, 2020 at 05:13:20PM -0700, Nick Desaulniers wrote:
> 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: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> .../Documentation/stack-validation.txt | 20 +++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
> index de094670050b..156fee13ba02 100644
> --- a/tools/objtool/Documentation/stack-validation.txt
> +++ b/tools/objtool/Documentation/stack-validation.txt
> @@ -289,6 +289,26 @@ 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_enable()
> +
> + These functions should 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.
> +
> + To fix, either:
> + 1) add the correct guards before and after calls to low level functions like
> + __get_user_size()/__put_user_size().
> + 2) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
> + funcB obviously does not call schedule().

There's sadly nothing obvious about 2); __fentry__ is enough to end up
in schedule() through preempt_enable().

So any function that has function tracing on (mostly everything) is
already disqualified.

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