linux-kernel - [PATCH v2] fs: Call kmap_local_page() in copy_string

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Date:   Sun, 24 Jul 2022 23:25:23 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH v2] 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 and not
globally visible. Furthermore, the mappings can be acquired from any
context (including interrupts).

Therefore, replace kmap_atomic() with kmap_local_page() in
copy_string_kernel(). Instead of open-coding local mapping + memcpy(),
use memcpy_to_page(). Delete a redundant call to flush_dcache_page().

Tested with xfstests on a QEMU/ KVM x86_32 VM, 6GB RAM, booting a kernel
with HIGHMEM64GB enabled.

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

v1->v2: Instead of open-coding a local mapping + memcpy(), use
memcpy_to_page() (thanks to Ira Weiny for his comment on v1).

 fs/exec.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index d20cb09476f3..eed899188333 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -630,7 +630,6 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
 		unsigned int bytes_to_copy = min_t(unsigned int, len,
 				min_not_zero(offset_in_page(pos), PAGE_SIZE));
 		struct page *page;
-		char *kaddr;

 		pos -= bytes_to_copy;
 		arg -= bytes_to_copy;
@@ -639,11 +638,8 @@ 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);
 		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);
+		memcpy_to_page(page, offset_in_page(pos), arg, bytes_to_copy);
 		put_arg_page(page);
 	}

-- 
2.37.1