[PATCH v2] fs: Call kmap_local_page() in copy_string_kernel()

[PATCH v2] fs: Call kmap_local_page() in copy_string_kernel()

[Fabio M. De Francesco]

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@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.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

Re: [PATCH v2] fs: Call kmap_local_page() in copy_string_kernel()

[Kees Cook]

On Sun, 24 Jul 2022 23:25:23 +0200, Fabio M. De Francesco wrote:
> 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().
> 
> [...]

Applied to for-next/execve, thanks!

[1/1] fs: Call kmap_local_page() in copy_string_kernel()
      https://git.kernel.org/kees/c/0ff95c390bc8

-- 
Kees Cook