linux-kernel - [PATCH v4 43/45] namei: initialize parameters passed to step

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Date:   Fri,  1 Jul 2022 16:23:08 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     glider@...gle.com
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Evgenii Stepanov <eugenis@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Vitaly Buka <vitalybuka@...gle.com>,
        linux-toolchains@...r.kernel.org
Subject: [PATCH v4 43/45] namei: initialize parameters passed to step_into()

Under certain circumstances initialization of `unsigned seq` and
`struct inode *inode` passed into step_into() may be skipped.
In particular, if the call to lookup_fast() in walk_component()
returns NULL, and lookup_slow() returns a valid dentry, then the
`seq` and `inode` will remain uninitialized until the call to
step_into() (see [1] for more info).

Right now step_into() does not use these uninitialized values,
yet passing uninitialized values to functions is considered undefined
behavior (see [2]). To fix that, we initialize `seq` and `inode` at
definition.

[1] https://github.com/ClangBuiltLinux/linux/issues/1648#issuecomment-1146608063
[2] https://lore.kernel.org/linux-toolchains/CAHk-=whjz3wO8zD+itoerphWem+JZz4uS3myf6u1Wd6epGRgmQ@mail.gmail.com/

Cc: Evgenii Stepanov <eugenis@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Marco Elver <elver@...gle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vitaly Buka <vitalybuka@...gle.com>
Cc: linux-kernel@...r.kernel.org
Cc: linux-toolchains@...r.kernel.org
Signed-off-by: Alexander Potapenko <glider@...gle.com>
---
Link: https://linux-review.googlesource.com/id/I94d4e8cc1f0ecc7174659e9506ce96aaf2201d0a
---
 fs/namei.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1f28d3f463c3b..6b39dfd3b41bc 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1995,8 +1995,8 @@ static const char *handle_dots(struct nameidata *nd, int type)
 static const char *walk_component(struct nameidata *nd, int flags)
 {
 	struct dentry *dentry;
-	struct inode *inode;
-	unsigned seq;
+	struct inode *inode = NULL;
+	unsigned seq = 0;
 	/*
 	 * "." and ".." are special - ".." especially so because it has
 	 * to be able to know about the current root directory and
@@ -3393,8 +3393,8 @@ static const char *open_last_lookups(struct nameidata *nd,
 	struct dentry *dir = nd->path.dentry;
 	int open_flag = op->open_flag;
 	bool got_write = false;
-	unsigned seq;
-	struct inode *inode;
+	unsigned seq = 0;
+	struct inode *inode = NULL;
 	struct dentry *dentry;
 	const char *res;
 
-- 
2.37.0.rc0.161.g10f37bed90-goog