[SRU Hirsute/Impish] vfs: check fd has read access in kernel_read_file_from_fd()

[Stefan Bader]

On 16.11.21 20:42, Thadeu Lima de Souza Cascardo wrote:
> From: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> 
> BugLink: https://bugs.launchpad.net/bugs/1950644
> 
> If we open a file without read access and then pass the fd to a syscall
> whose implementation calls kernel_read_file_from_fd(), we get a warning
> from __kernel_read():
> 
>          if (WARN_ON_ONCE(!(file->f_mode & FMODE_READ)))
> 
> This currently affects both finit_module() and kexec_file_load(), but it
> could affect other syscalls in the future.
> 
> Link: https://lkml.kernel.org/r/20211007220110.600005-1-willy@infradead.org
> Fixes: b844f0ecbc56 ("vfs: define kernel_copy_file_from_fd()")
> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
> Reported-by: Hao Sun <sunhao.th at gmail.com>
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Acked-by: Christian Brauner <christian.brauner at ubuntu.com>
> Cc: Al Viro <viro at zeniv.linux.org.uk>
> Cc: Mimi Zohar <zohar at linux.ibm.com>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (cherry picked from commit 032146cda85566abcd1c4884d9d23e4e30a07e9a)
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> ---

There are 2 duplicates caused by me moderating the mailing list. Thadeu, not 
sure I really see why one was blocked and the other not. Just generally if you 
could delay re-sends until the next day. Do not count on my ability to realize 
what was already re-send. Hard enough to say what is spam or not sometimes...

-Stefan

>   fs/kernel_read_file.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/kernel_read_file.c b/fs/kernel_read_file.c
> index 90d255fbdd9b..c84d87f558cb 100644
> --- a/fs/kernel_read_file.c
> +++ b/fs/kernel_read_file.c
> @@ -178,7 +178,7 @@ int kernel_read_file_from_fd(int fd, loff_t offset, void **buf,
>   	struct fd f = fdget(fd);
>   	int ret = -EBADF;
>   
> -	if (!f.file)
> +	if (!f.file || !(f.file->f_mode & FMODE_READ))
>   		goto out;
>   
>   	ret = kernel_read_file(f.file, offset, buf, buf_size, file_size, id);
> 


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