Hi,
On 23.1.2024 10.13, Geert Uytterhoeven wrote:
On Tue, Jan 23, 2024 at 1:35 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:
VAR + value < VAR
Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.
Refactor open-coded unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). This paves the way to enabling the
unsigned wrap-around sanitizer[2] in the future.
Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Liam Howlett <liam.howlett@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Thanks for your patch!
--- a/arch/m68k/kernel/sys_m68k.c
+++ b/arch/m68k/kernel/sys_m68k.c
@@ -391,10 +391,11 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
mmap_read_lock(current->mm);
} else {
+ unsigned long sum;
"sum" sounds like this is a dummy variable, to please the third
parameter of check_add_overflow()...
struct vm_area_struct *vma;
/* Check for overflow. */
I agree with Liam: please drop the comment.
- if (addr + len < addr)
+ if (check_add_overflow(addr, len, &sum))
goto out;
/*
@@ -403,7 +404,7 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
*/
mmap_read_lock(current->mm);
vma = vma_lookup(current->mm, addr);
- if (!vma || addr + len > vma->vm_end)
+ if (!vma || sum > vma->vm_end)
... Oh, it is actually used. What about renaming it to "end" instead?
IMHO this is more descriptive:
+ if (check_add_overflow(addr, len, &sum))
than this:
+ if (check_add_overflow(addr, len, &end))
"sum" is IMHO quite obviously sum of the preceding args, whereas I do
not know what "end" would be.
- Eero
goto out_unlock;
}
With the above fixed:
Reviewed-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Acked-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
If you want me to take this through the m68k tree (for v6.9), please
let me know.
Thanks!
Gr{oetje,eeting}s,
Geert
