Privileged access to files and directories is actually determined by capabilities, not just by being root or not. In practice, root usually has all possible capabilities, but there are situations where all/many of them could be dropped, or some given to other users (their processes).
In brief, you already described how the access control checks work for a privileged process. Here's how the different capabilities actually affect it:
The main capability here is CAP_DAC_OVERRIDE, a process that has it can "bypass file read, write, and execute permission checks". That includes reading and writing to any files, as well as reading, writing and accessing directories.
It doesn't actually apply to executing files that are not marked as executable. The comment in generic_permission (fs/namei.c), before the access checks for files, says that
Read/write DACs are always overridable. Executable DACs are overridable when there is at least one exec bit set.
And the code checks that there's at least one x bit set if you're trying to execute the file. I suspect that's only a convenience feature, to prevent accidentally running random data files and getting errors or odd results.
Anyway, if you can override permissions, you could just make an executable copy and run that. (Though it might make a difference in theory for setuid files of a process was capable of overriding file permissions (CAP_DAC_OVERRIDE), but didn't have other related capabilities (CAP_FSETID/CAP_FOWNER/CAP_SETUID). But having CAP_DAC_OVERRIDE allows editing /etc/shadow and stuff like that, so it's approximately equal to just having full root access anyway.)
There's also the CAP_DAC_READ_SEARCH capability that allows to read any files and access any directories, but not to execute or write to them; and CAP_FOWNER that allows a process to do stuff that's usually reserved only for the file owner, like changing the permission bits and file group.
Overriding the sticky bit on directories is mentioned only under CAP_FOWNER, so it seems that CAP_DAC_OVERRIDE would not be enough to ignore that. (It would give you write permission, but usually in sticky directories you have that anyway, and +t limits it.)
(I think special devices count as "files" here. At least generic_permission() only has a type check for directories, but I didn't check outside of that.)
Of course, there are still situations where even capabilities will not help you modify files:
- some files in
/proc and /sys, since they're not really actual files
- SELinux and other security modules that might limit root
chattr immutable +i and append only +a flags on ext2/ext3/ext4, both of which stop even root, and prevent also file renames etc.- network filesystems, where the server can do its own access control, e.g.
root_squash in NFS maps root to nobody
- FUSE, which I assume could do anything
- read-only mounts
- read-only devices
Best Answer
The rules are as following:
So for root it doesn't matter much anyway - but for other users, it's the most specific permission-triplet that applies. So in your example, if the user is both the owner and a member of the group, it's the owner-triplet that's used (not the group-triplet). So if the group got both read-and-write-permission, but the owner only got read-permission; then the owner will only be allowed to read the file - even though his group-membership ought to let him write to it too. Other (non-owner) members of the group, will be allowed to both read-and-write the file.
The owner of a file can always add more permission to himself if he needs to - and sometimes a program may do it for you. For example, if you got a write-protected file (eg. permission r--r-----), some editors will allow you to write to them anyway (usually after a confirmation). The editor is running as you, and as you own the file and can change its permissions, the editor can remove write-protection and allow you to save the file.
+++
It means that it's the root-user who owns the file and got permission to both read and write it - the owner (root) may also change the file's permission. And that members of the root-group are allowed to read it. Other users can neither read, write nor execute the file. (Since it's a text-file, it's probably little point of executing it anyway.)
Many files on a Linux-system got root-user as it's owner and root-group as it's group. Although, traditionally various system-users and system-groups - like bin, sys, proc, operator - owned many files rather than root. For example, the binaries (the executable programs) usually had bin-user and/or bin-group as ownership (eg. bin:bin or root:bin).
The exception to this was executables that had to run as root - they had to be owned by the root-user. Usually programs execute as/with the permission of whomever user executed the program. If you run the command
ls, it runs with your permissions, and therefore cannot show directories you're not allowed to list (like the directories of other users). If a command is run with root-permission on the other hand, it got access to the whole system (which is why you don't want to that on most executables).One good is example is the
passwd-command which lets you change password. This is run as root, and gives any user limited access to the files used to store the user-and password-databases.rwsr-xr-x root:root /usr/bin/passwd
s=x+S, where x is execute-permission, and S is run as owner or run as group, depending if it's set for the owner or group triplet.
So root-user is the owner; and got read, write and execute permission. root-group is the group, and got read and execute permission. While other users also got read and execute permissions. In addition, the executable will run with the permissions of it's owner - ie. root - and not with the permissions of the user executing it (as is normal), thanks to the "s" (u+s) in the owner-triplet.
Another example, this time from BSD (a UNIX OS):
rws-x--- root:wheel /bin/su
This means the the executable
suis always run as it's owner - root. That root-user may read, write and execute it. That members of the wheel-group is allwoed to execute it, but not to read (eg. copy) it. And that other users may neither read, write nor execute it. (The commandsuexists on Linux also, but here all users may execute it - it still runs as the root-user though.)Other programs may also run as some system-user (and group) - for example the
apacheweb-server is often run as the www-data-user (and www-data-group). This way it can't do too much damage if compromised, due to lack of permissions where it doesn't belong.