Re: [PATCH v7 3/7] exec: Move path_noexec() check earlier

[Mickaël Salaün]

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@xxxxxxxxxxx> writes:
> 
>> From: Kees Cook <keescook@xxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxx/

> 
> 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@xxxxxxxxxxx>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@xxxxxxxxxxxx
>> ---
>>  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);