On Mon, Jun 24, 2019 at 7:50 PM Catalin Marinas [off-list ref] wrote:

On Mon, Jun 24, 2019 at 04:32:52PM +0200, Andrey Konovalov wrote:

quoted

This patch is a part of a series that extends kernel ABI to allow to pass
tagged user pointers (with the top byte set to something else other than
0x00) as syscall arguments.

In copy_mount_options a user address is being subtracted from TASK_SIZE.
If the address is lower than TASK_SIZE, the size is calculated to not
allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
However if the address is tagged, then the size will be calculated
incorrectly.

Untag the address before subtracting.

Reviewed-by: Khalid Aziz <redacted>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Kees Cook <redacted>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrey Konovalov <redacted>
---
 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7660c2749c96..ec78f7223917 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -2994,7 +2994,7 @@ void *copy_mount_options(const void __user * data)
       * the remainder of the page.
       */
      /* copy_from_user cannot cross TASK_SIZE ! */
-     size = TASK_SIZE - (unsigned long)data;
+     size = TASK_SIZE - (unsigned long)untagged_addr(data);
      if (size > PAGE_SIZE)
              size = PAGE_SIZE;

I think this patch needs an ack from Al Viro (cc'ed).

--
Catalin

Hi Al,

Could you take a look and give your acked-by?

Thanks!

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