Hi Miklos, We've detected a suspicious double-fetch of user-space data in ovl_ioctl_set_flags using a prototype tool (see report below [1]). It points to ovl_ioctl_set_flags that does a capability check using flags, but then the real ioctl double-fetches flags and uses potentially different value: static long ovl_ioctl_set_flags(struct file *file, unsigned int cmd, unsigned long arg, unsigned int flags) { ... /* Check the capability before cred override */ oldflags = ovl_iflags_to_fsflags(READ_ONCE(inode->i_flags)); ret = vfs_ioc_setflags_prepare(inode, oldflags, flags); if (ret) goto unlock; ... ret = ovl_real_ioctl(file, cmd, arg); All fs impls call vfs_ioc_setflags_prepare again, so the capability is checked again. But I think this makes the vfs_ioc_setflags_prepare check in overlayfs pointless (?) and the "Check the capability before cred override" comment misleading, user can skip this check by presenting benign flags first and then overwriting them to non-benign flags. Or, if this check is still needed... it is wrong (?). The code would need to arrange for both ioctl's to operate on the same data then. Does it make any sense? Thanks [1] BUG: multi-read in __x64_sys_ioctl between ovl_ioctl and ext4_ioctl ======= First Address Range Stack ======= df_save_stack+0x33/0x70 lib/df-detection.c:208 add_address+0x2ac/0x352 lib/df-detection.c:47 ovl_ioctl_set_fsflags fs/overlayfs/file.c:607 [inline] ovl_ioctl+0x7d/0x290 fs/overlayfs/file.c:654 vfs_ioctl fs/ioctl.c:48 [inline] __do_sys_ioctl fs/ioctl.c:753 [inline] __se_sys_ioctl fs/ioctl.c:739 [inline] __x64_sys_ioctl+0xfc/0x140 fs/ioctl.c:739 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46 entry_SYSCALL_64_after_hwframe+0x44/0xa9 ======= Second Address Range Stack ======= df_save_stack+0x33/0x70 lib/df-detection.c:208 add_address+0x2ac/0x352 lib/df-detection.c:47 ext4_ioctl+0x13b1/0x27f0 fs/ext4/ioctl.c:833 vfs_ioctl+0x30/0x80 fs/ioctl.c:48 ovl_real_ioctl+0xed/0x100 fs/overlayfs/file.c:539 ovl_ioctl_set_flags+0x11d/0x180 fs/overlayfs/file.c:574 ovl_ioctl_set_fsflags fs/overlayfs/file.c:610 [inline] ovl_ioctl+0x11e/0x290 fs/overlayfs/file.c:654 vfs_ioctl fs/ioctl.c:48 [inline] __do_sys_ioctl fs/ioctl.c:753 [inline] __se_sys_ioctl fs/ioctl.c:739 [inline] __x64_sys_ioctl+0xfc/0x140 fs/ioctl.c:739 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46 entry_SYSCALL_64_after_hwframe+0x44/0xa9 syscall number 16 System Call: __x64_sys_ioctl+0x0/0x140 fs/ioctl.c:800 First 0000000020000000 len 4 Caller vfs_ioctl fs/ioctl.c:48 [inline] First 0000000020000000 len 4 Caller __do_sys_ioctl fs/ioctl.c:753 [inline] First 0000000020000000 len 4 Caller __se_sys_ioctl fs/ioctl.c:739 [inline] First 0000000020000000 len 4 Caller __x64_sys_ioctl+0xfc/0x140 fs/ioctl.c:739 Second 0000000020000000 len 4 Caller vfs_ioctl+0x30/0x80 fs/ioctl.c:48 ==================================================================