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 <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
---
.../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().
+
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.
--
2.25.1.696.g5e7596f4ac-goog
On 3/23/20 5:13 PM, 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 <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
> ---
> .../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
should be called
> + 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().
> +
>
> 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.
>
--
~Randy
On 3/24/20 5:43 AM, 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 <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
Looks good, a minor nitpick below.
> ---
> .../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()
^^^^^^
uaccess_disable()
> +
> + These functions should called to denote a minimal critical section around
> + access to __user variables. See also: https://lwn.net/Articles/517475/
--
Kamalesh
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 <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
> ---
> .../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.
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 <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
> ---
> .../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()
I think you meant to put "disable" here ^^^^^^
Cheers,
-Matt Helsley
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 <[email protected]>
Suggested-by: Matt Helsley <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
Suggested-by: Randy Dunlap <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
---
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
On Thu, Mar 26, 2020 at 11:37:06AM -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: Kamalesh Babulal <[email protected]>
> Suggested-by: Matt Helsley <[email protected]>
> Suggested-by: Peter Zijlstra <[email protected]>
> Suggested-by: Randy Dunlap <[email protected]>
> Signed-off-by: Nick Desaulniers <[email protected]>
> ---
> 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..."
Thanks, adding this one to the queue for tip.
--
Josh