On Fri, Nov 13, 2020 at 09:01:32AM +0100, Roberto Sassu wrote:
> Commit a1f9b1c0439db ("integrity/ima: switch to using __kernel_read")
> replaced the __vfs_read() call in integrity_kernel_read() with
> __kernel_read(), a new helper introduced by commit 61a707c543e2a ("fs: add
> a __kernel_read helper").
>
> Since the new helper requires that also the FMODE_CAN_READ flag is set in
> file->f_mode, this patch saves the original f_mode and sets the flag if the
> the file descriptor has the necessary file operation. Lastly, it restores
> the original f_mode at the end of ima_calc_file_hash().
This looks bogus. FMODE_CAN_READ has a pretty clear definition and
you can't just go and read things if it is not set. Also f_mode
manipulations on a life file are racy.
>
> Cc: [email protected] # 5.8.x
> Fixes: a1f9b1c0439db ("integrity/ima: switch to using __kernel_read")
> Signed-off-by: Roberto Sassu <[email protected]>
> ---
> security/integrity/ima/ima_crypto.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/security/integrity/ima/ima_crypto.c
> b/security/integrity/ima/ima_crypto.c
> index 21989fa0c107..22ed86a0c964 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -537,6 +537,7 @@ int ima_calc_file_hash(struct file *file, struct
> ima_digest_data *hash)
> loff_t i_size;
> int rc;
> struct file *f = file;
> + fmode_t saved_mode;
> bool new_file_instance = false, modified_mode = false;
>
> /*
> @@ -550,7 +551,7 @@ int ima_calc_file_hash(struct file *file, struct
> ima_digest_data *hash)
> }
>
> /* Open a new file instance in O_RDONLY if we cannot read */
> - if (!(file->f_mode & FMODE_READ)) {
> + if (!(file->f_mode & FMODE_READ) || !(file->f_mode & FMODE_CAN_READ)) {
> int flags = file->f_flags & ~(O_WRONLY | O_APPEND |
> O_TRUNC | O_CREAT | O_NOCTTY | O_EXCL);
> flags |= O_RDONLY;
> @@ -562,7 +563,10 @@ int ima_calc_file_hash(struct file *file, struct
> ima_digest_data *hash)
> */
> pr_info_ratelimited("Unable to reopen file for
> reading.\n");
> f = file;
> + saved_mode = f->f_mode;
> f->f_mode |= FMODE_READ;
> + if (likely(file->f_op->read || file->f_op->read_iter))
> + f->f_mode |= FMODE_CAN_READ;
> modified_mode = true;
> } else {
> new_file_instance = true;
> @@ -582,7 +586,7 @@ int ima_calc_file_hash(struct file *file, struct
> ima_digest_data *hash)
> if (new_file_instance)
> fput(f);
> else if (modified_mode)
> - f->f_mode &= ~FMODE_READ;
> + f->f_mode = saved_mode;
> return rc;
> }
>
> --
> 2.27.GIT
>
---end quoted text---
