linux-kernel - [PATCH] fs: Call kmap_local_page() in copy_string

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Date:   Sun, 10 Jul 2022 12:01:36 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Benjamin LaHaise <bcrl@...ck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, nvdimm@...ts.linux.dev,
        io-uring@...r.kernel.org, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] fs: Call kmap_local_page() in copy_string_kernel()

The use of kmap_atomic() is being deprecated in favor of kmap_local_page().

With kmap_local_page(), the mappings are per thread, CPU local, not
globally visible and can take page faults. Furthermore, the mappings can be
acquired from any context (including interrupts).

Therefore, use kmap_local_page() in copy_string_kernel() instead of
kmap_atomic().

Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
HIGHMEM64GB enabled.

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

I sent a first patch to fs/exec.c for converting kmap() and kmap_atomic()
to kmap_local_page():
https://lore.kernel.org/lkml/20220630163527.9776-1-fmdefrancesco@gmail.com/

Some days ago, Ira Weiny, while he was reviewing that patch, made me notice
that I had overlooked a second kmap_atomic() in the same file (thanks):
https://lore.kernel.org/lkml/YsiQptk19txHrG4c@iweiny-desk3/

I've been asked to send this as an additional change. This is why there will
not be any second version of that previous patch.

 fs/exec.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 4a2129c0d422..5fa652ca5823 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -639,11 +639,11 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
 		page = get_arg_page(bprm, pos, 1);
 		if (!page)
 			return -E2BIG;
-		kaddr = kmap_atomic(page);
+		kaddr = kmap_local_page(page);
 		flush_arg_page(bprm, pos & PAGE_MASK, page);
 		memcpy(kaddr + offset_in_page(pos), arg, bytes_to_copy);
 		flush_dcache_page(page);
-		kunmap_atomic(kaddr);
+		kunmap_local(kaddr);
 		put_arg_page(page);
 	}
 
-- 
2.36.1