[v7,3/7] exec: Move path_noexec() check earlier
diff mbox series

Message ID 20200723171227.446711-4-mic@digikod.net
State New
Headers show
Series
  • Add support for O_MAYEXEC
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Commit Message

Mickaël Salaün July 23, 2020, 5:12 p.m. UTC
From: Kees Cook <keescook@chromium.org>

The path_noexec() check, like the regular file check, was happening too
late, letting LSMs see impossible execve()s. Check it earlier as well
in may_open() and collect the redundant fs/exec.c path_noexec() test
under the same robustness comment as the S_ISREG() check.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
    struct open_flags open_exec_flags = {
        .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
        .acc_mode = MAY_EXEC,
        ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
                may_open(path, acc_mode, open_flag)
                    /* new location of MAY_EXEC vs path_noexec() test */
                    inode_permission(inode, MAY_OPEN | acc_mode)
                        security_inode_permission(inode, acc_mode)
                vfs_open(path, file)
                    do_dentry_open(file, path->dentry->d_inode, open)
                        security_file_open(f)
                        open()
    /* old location of path_noexec() test */

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
---
 fs/exec.c  | 12 ++++--------
 fs/namei.c |  4 ++++
 2 files changed, 8 insertions(+), 8 deletions(-)

Comments

Eric W. Biederman Aug. 11, 2020, 7:36 p.m. UTC | #1
Mickaël Salaün <mic@digikod.net> writes:

> From: Kees Cook <keescook@chromium.org>
>
> The path_noexec() check, like the regular file check, was happening too
> late, letting LSMs see impossible execve()s. Check it earlier as well
> in may_open() and collect the redundant fs/exec.c path_noexec() test
> under the same robustness comment as the S_ISREG() check.
>
> My notes on the call path, and related arguments, checks, etc:

A big question arises, that I think someone already asked.

Why perform this test in may_open directly instead of moving
it into inode_permission.  That way the code can be shared with
faccessat, and any other code path that wants it?

That would look to provide a more maintainable kernel.

Eric


> do_open_execat()
>     struct open_flags open_exec_flags = {
>         .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>         .acc_mode = MAY_EXEC,
>         ...
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             file = alloc_empty_file(open_flags, current_cred());
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
>                     /* new location of MAY_EXEC vs path_noexec() test */
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         security_file_open(f)
>                         open()
>     /* old location of path_noexec() test */
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
> ---
>  fs/exec.c  | 12 ++++--------
>  fs/namei.c |  4 ++++
>  2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index bdc6a6eb5dce..4eea20c27b01 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  	 * and check again at the very end too.
>  	 */
>  	error = -EACCES;
> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> -		goto exit;
> -
> -	if (path_noexec(&file->f_path))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> +			 path_noexec(&file->f_path)))
>  		goto exit;
>  
>  	fsnotify_open(file);
> @@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	 * and check again at the very end too.
>  	 */
>  	err = -EACCES;
> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> -		goto exit;
> -
> -	if (path_noexec(&file->f_path))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> +			 path_noexec(&file->f_path)))
>  		goto exit;
>  
>  	err = deny_write_access(file);
> diff --git a/fs/namei.c b/fs/namei.c
> index a559ad943970..ddc9b25540fe 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>  			return -EACCES;
>  		flag &= ~O_TRUNC;
>  		break;
> +	case S_IFREG:
> +		if ((acc_mode & MAY_EXEC) && path_noexec(path))
> +			return -EACCES;
> +		break;
>  	}
>  
>  	error = inode_permission(inode, MAY_OPEN | acc_mode);
Mickaël Salaün Aug. 13, 2020, 3:31 p.m. UTC | #2
Kees Cook wrote this patch, which is in Andrew Morton's tree, but I
think you're talking about O_MAYEXEC, not this patch specifically.

On 11/08/2020 21:36, Eric W. Biederman wrote:
> Mickaël Salaün <mic@digikod.net> writes:
> 
>> From: Kees Cook <keescook@chromium.org>
>>
>> The path_noexec() check, like the regular file check, was happening too
>> late, letting LSMs see impossible execve()s. Check it earlier as well
>> in may_open() and collect the redundant fs/exec.c path_noexec() test
>> under the same robustness comment as the S_ISREG() check.
>>
>> My notes on the call path, and related arguments, checks, etc:
> 
> A big question arises, that I think someone already asked.

Al Viro and Jann Horn expressed such concerns for O_MAYEXEC:
https://lore.kernel.org/lkml/0cc94c91-afd3-27cd-b831-8ea16ca8ca93@digikod.net/

> 
> Why perform this test in may_open directly instead of moving
> it into inode_permission.  That way the code can be shared with
> faccessat, and any other code path that wants it?

This patch is just a refactoring.

About O_MAYEXEC, path-based LSM, IMA and IPE need to work on a struct
file, whereas inode_permission() only gives a struct inode. However,
faccessat2(2) (with extended flags) seems to be the perfect candidate if
we want to be able to check file descriptors.

> 
> That would look to provide a more maintainable kernel.

Why would it be more maintainable?

> 
> Eric
> 
> 
>> do_open_execat()
>>     struct open_flags open_exec_flags = {
>>         .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>>         .acc_mode = MAY_EXEC,
>>         ...
>>     do_filp_open(dfd, filename, open_flags)
>>         path_openat(nameidata, open_flags, flags)
>>             file = alloc_empty_file(open_flags, current_cred());
>>             do_open(nameidata, file, open_flags)
>>                 may_open(path, acc_mode, open_flag)
>>                     /* new location of MAY_EXEC vs path_noexec() test */
>>                     inode_permission(inode, MAY_OPEN | acc_mode)
>>                         security_inode_permission(inode, acc_mode)
>>                 vfs_open(path, file)
>>                     do_dentry_open(file, path->dentry->d_inode, open)
>>                         security_file_open(f)
>>                         open()
>>     /* old location of path_noexec() test */
>>
>> Signed-off-by: Mickaël Salaün <mic@digikod.net>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
>> ---
>>  fs/exec.c  | 12 ++++--------
>>  fs/namei.c |  4 ++++
>>  2 files changed, 8 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index bdc6a6eb5dce..4eea20c27b01 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>>  	 * and check again at the very end too.
>>  	 */
>>  	error = -EACCES;
>> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>> -		goto exit;
>> -
>> -	if (path_noexec(&file->f_path))
>> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
>> +			 path_noexec(&file->f_path)))
>>  		goto exit;
>>  
>>  	fsnotify_open(file);
>> @@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>>  	 * and check again at the very end too.
>>  	 */
>>  	err = -EACCES;
>> -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>> -		goto exit;
>> -
>> -	if (path_noexec(&file->f_path))
>> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
>> +			 path_noexec(&file->f_path)))
>>  		goto exit;
>>  
>>  	err = deny_write_access(file);
>> diff --git a/fs/namei.c b/fs/namei.c
>> index a559ad943970..ddc9b25540fe 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>>  			return -EACCES;
>>  		flag &= ~O_TRUNC;
>>  		break;
>> +	case S_IFREG:
>> +		if ((acc_mode & MAY_EXEC) && path_noexec(path))
>> +			return -EACCES;
>> +		break;
>>  	}
>>  
>>  	error = inode_permission(inode, MAY_OPEN | acc_mode);

Patch
diff mbox series

diff --git a/fs/exec.c b/fs/exec.c
index bdc6a6eb5dce..4eea20c27b01 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -147,10 +147,8 @@  SYSCALL_DEFINE1(uselib, const char __user *, library)
 	 * and check again at the very end too.
 	 */
 	error = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
-		goto exit;
-
-	if (path_noexec(&file->f_path))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+			 path_noexec(&file->f_path)))
 		goto exit;
 
 	fsnotify_open(file);
@@ -897,10 +895,8 @@  static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	 * and check again at the very end too.
 	 */
 	err = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
-		goto exit;
-
-	if (path_noexec(&file->f_path))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+			 path_noexec(&file->f_path)))
 		goto exit;
 
 	err = deny_write_access(file);
diff --git a/fs/namei.c b/fs/namei.c
index a559ad943970..ddc9b25540fe 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2863,6 +2863,10 @@  static int may_open(const struct path *path, int acc_mode, int flag)
 			return -EACCES;
 		flag &= ~O_TRUNC;
 		break;
+	case S_IFREG:
+		if ((acc_mode & MAY_EXEC) && path_noexec(path))
+			return -EACCES;
+		break;
 	}
 
 	error = inode_permission(inode, MAY_OPEN | acc_mode);