Ever since idmapped mounts were introduced, inode ownership checks (for side-channel protection) in mincore() and madvise(MADV_PAGEOUT) were done against the nop_mnt_idmap, which completely ignores the file's mount's idmap. This results in odd edgecases like: 1) mount/bind-mount with an idmap userA:userB:1 2) userB runs an owner_or_capable() check on file that is owned by userA on-disk/in-memory, but owned by userB after idmap translation 3) owner_or_capable() mysteriously fails as the correct idmap wasn't supplied In the case of mincore/madvise MADV_PAGEOUT, this is usually benign, because file_permission(file, MAY_WRITE) will probably succeed, as it uses the proper idmap internally, but it does not need to be the case on e.g a 0444 file where even the owner itself doesn't have permissions to write to it. Since this is clearly not trivial to get right, introduce a file_owner_or_capable() that can carry the correct semantics, and switch the various users in mm to it. The issue was found by manual code inspection & an off-list discussion with Jan Kara.
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| @@ -2444,6 +2444,11 @@ static inline struct mnt_idmap *file_mnt_idmap(const struct file *file) · 2443 unmodified lines | ||
| 2444 | 2444 | return mnt_idmap(file->f_path.mnt); |
| 2445 | 2445 | } |
| 2446 | 2446 | |
| 2447 | +static inline bool file_owner_or_capable(const struct file *file) | |
| 2448 | +{ | |
| 2449 | + return inode_owner_or_capable(file_mnt_idmap(file), file_inode(file)); | |
| 2450 | +} | |
| 2451 | + | |
| 2447 | 2452 | /** |
| 2448 | 2453 | * is_idmapped_mnt - check whether a mount is mapped |
| 2449 | 2454 | * @mnt: the mount to check |