Re: [PATCH 2/3] open: don't silently ignore unknown O-flags in openat2()

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2021-04-23, Christian Brauner <brauner@kernel.org> wrote:
> The new openat2() syscall verifies that no unknown O-flag values are
> set and returns an error to userspace if they are while the older open
> syscalls like open() and openat2() simply ignore unknown flag values:
> 
>   #define O_FLAG_CURRENTLY_INVALID (1 << 31)
>   struct open_how how = {
>           .flags = O_RDONLY | O_FLAG_CURRENTLY_INVALID,
>           .resolve = 0,
>   };
> 
>   /* fails */
>   fd = openat2(-EBADF, "/dev/null", &how, sizeof(how));
> 
>   /* succeeds */
>   fd = openat(-EBADF, "/dev/null", O_RDONLY | O_FLAG_CURRENTLY_INVALID);
> 
> However, openat2() silently truncates the upper 32 bits meaning:
> 
>   #define O_FLAG_CURRENTLY_INVALID_LOWER32 (1 << 31)
>   #define O_FLAG_CURRENTLY_INVALID_UPPER32 (1 << 40)
> 
>   struct open_how how_lowe32 = {
>           .flags = O_RDONLY | O_FLAG_CURRENTLY_INVALID_LOWE32,
>           .resolve = 0,
>   };
> 
>   struct open_how how_upper32 = {
>           .flags = O_RDONLY | O_FLAG_CURRENTLY_INVALID_LOWE32,
>           .resolve = 0,
>   };
> 
>   /* fails */
>   fd = openat2(-EBADF, "/dev/null", &how_lower32, sizeof(how_lower32));
> 
>   /* succeeds */
>   fd = openat2(-EBADF, "/dev/null", &how_upper32, sizeof(how_upper32));
> 
> That seems like a bug. Fix it by preventing the truncation in
> build_open_flags().
> 
> There's a snafu here though stripping FMODE_* directly from flags would
> cause the upper 32 bits to be truncated as well due to integer promotion
> rules since FMODE_* is unsigned int, O_* are signed ints (yuck).
> 
> This change shouldn't regress old open syscalls since they silently
> truncate any unknown values.

Yeah, oops on my part (I was always worried I'd missed something with
making everything -EINVAL).

Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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