RE: [RESEND][PATCH] ima: Set and clear FMODE_CAN_READ in ima_calc_file_hash()

From: Christoph Hellwig [mailto:hch@infradead.org]
Sent: Saturday, November 14, 2020 12:11 PM
On Fri, Nov 13, 2020 at 09:01:32AM +0100, Roberto Sassu wrote:

quoted

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

quoted

a __kernel_read helper").

Since the new helper requires that also the FMODE_CAN_READ flag is set

in

quoted

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.

FMODE_CAN_READ was not set because f_mode does not have
FMODE_READ. In the patch, I check if the former can be set
similarly to the way it is done in file_table.c and open.c.

Is there a better way to read a file when the file was not opened
for reading and a new file descriptor cannot be created?

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

quoted

Cc: <redacted> # 5.8.x
Fixes: a1f9b1c0439db ("integrity/ima: switch to using __kernel_read")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 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

quoted

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)

quoted

 	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)

quoted

 	}

 	/* 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)) {

quoted

 		int flags = file->f_flags & ~(O_WRONLY | O_APPEND |
 				O_TRUNC | O_CREAT | O_NOCTTY |

O_EXCL);

quoted

 		flags |= O_RDONLY;

@@ -562,7 +563,10 @@ int ima_calc_file_hash(struct file *file, struct

ima_digest_data *hash)

quoted

 			 */
 			pr_info_ratelimited("Unable to reopen file for

reading.\n");

quoted

 			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)

quoted

 	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---