From: Nick Desaulniers <ndesaulniers@google.com>
To: jpoimboe@redhat.com, peterz@infradead.org
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
Wolfram Sang <wsa@the-dreams.de>,
Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
Raphael Gault <raphael.gault@arm.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: [PATCH] objtool: Documentation: document UACCESS warnings
Date: Mon, 23 Mar 2020 17:13:20 -0700 [thread overview]
Message-ID: <20200324001321.39562-1-ndesaulniers@google.com> (raw)
In-Reply-To: <20200323212538.GN2452@worktop.programming.kicks-ass.net>
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().
+
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
next parent reply other threads:[~2020-03-24 0:13 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200323212538.GN2452@worktop.programming.kicks-ass.net>
2020-03-24 0:13 ` Nick Desaulniers [this message]
2020-03-24 1:12 ` [PATCH] objtool: Documentation: document UACCESS warnings Randy Dunlap
2020-03-24 8:41 ` Kamalesh Babulal
2020-03-24 12:52 ` Peter Zijlstra
2020-03-26 13:47 ` Matt Helsley
2020-03-26 18:37 ` [PATCH v2] " Nick Desaulniers
2020-04-03 16:20 ` Josh Poimboeuf
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