https://git.kernel.org/torvalds/c/299e2b1967578b1442128ba8b3e86ed3427d3651 Signed-off-by: Günther Noack <gnoack3000@xxxxxxxxx> Reviewed-by: Mickaël Salaün <mic@xxxxxxxxxxx> --- man7/landlock.7 | 83 +++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 81 insertions(+), 2 deletions(-) diff --git a/man7/landlock.7 b/man7/landlock.7 index d3a7ec0d2..9c305edef 100644 --- a/man7/landlock.7 +++ b/man7/landlock.7 @@ -64,9 +64,39 @@ Execute a file. .TP .B LANDLOCK_ACCESS_FS_WRITE_FILE Open a file with write access. +.IP +When opening files for writing, +you will often additionally need the +.B LANDLOCK_ACCESS_FS_TRUNCATE +right. +In many cases, +these system calls truncate existing files when overwriting them +(e.g., +.BR creat (2)). .TP .B LANDLOCK_ACCESS_FS_READ_FILE Open a file with read access. +.TP +.B LANDLOCK_ACCESS_FS_TRUNCATE +Truncate a file with +.BR truncate (2), +.BR ftruncate (2), +.BR creat (2), +or +.BR open (2) +with +.BR O_TRUNC . +Whether an opened file can be truncated with +.BR ftruncate (2) +is determined during +.BR open (2), +in the same way as read and write permissions are checked during +.BR open (2) +using +.B LANDLOCK_ACCESS_FS_READ_FILE +and +.BR LANDLOCK_ACCESS_FS_WRITE_FILE . +This access right is available since the third version of the Landlock ABI. .PP A directory can receive access rights related to files or directories. The following access right is applied to the directory itself, @@ -231,6 +261,53 @@ To be allowed to use and related syscalls on a target process, a sandboxed process should have a subset of the target process rules, which means the tracee must be in a sub-domain of the tracer. +.\" +.SS Truncating files +The operations covered by +.B LANDLOCK_ACCESS_FS_WRITE_FILE +and +.B LANDLOCK_ACCESS_FS_TRUNCATE +both change the contents of a file and sometimes overlap in +non-intuitive ways. +It is recommended to always specify both of these together. +.PP +A particularly surprising example is +.BR creat (2). +The name suggests that this system call requires +the rights to create and write files. +However, it also requires the truncate right +if an existing file under the same name is already present. +.PP +It should also be noted that truncating files does not require the +.B LANDLOCK_ACCESS_FS_WRITE_FILE +right. +Apart from the +.BR truncate (2) +system call, this can also be done through +.BR open (2) +with the flags +.IR "O_RDONLY\ |\ O_TRUNC" . +.PP +When opening a file, the availability of the +.B LANDLOCK_ACCESS_FS_TRUNCATE +right is associated with the newly created file descriptor +and will be used for subsequent truncation attempts using +.BR ftruncate (2). +The behavior is similar to opening a file for reading or writing, +where permissions are checked during +.BR open (2), +but not during the subsequent +.BR read (2) +and +.BR write (2) +calls. +.PP +As a consequence, +it is possible to have multiple open file descriptors for the same file, +where one grants the right to truncate the file and the other does not. +It is also possible to pass such file descriptors between processes, +keeping their Landlock properties, +even when these processes do not have an enforced Landlock ruleset. .SH VERSIONS Landlock was introduced in Linux 5.13. .PP @@ -257,6 +334,8 @@ _ _ _ \^ \^ LANDLOCK_ACCESS_FS_MAKE_SYM _ _ _ 2 5.19 LANDLOCK_ACCESS_FS_REFER +_ _ _ +3 6.2 LANDLOCK_ACCESS_FS_TRUNCATE .TE .sp 1 .PP @@ -302,7 +381,6 @@ in kernel logs. It is currently not possible to restrict some file-related actions accessible through these system call families: .BR chdir (2), -.BR truncate (2), .BR stat (2), .BR flock (2), .BR chmod (2), @@ -340,7 +418,8 @@ attr.handled_access_fs = LANDLOCK_ACCESS_FS_MAKE_FIFO | LANDLOCK_ACCESS_FS_MAKE_BLOCK | LANDLOCK_ACCESS_FS_MAKE_SYM | - LANDLOCK_ACCESS_FS_REFER; + LANDLOCK_ACCESS_FS_REFER | + LANDLOCK_ACCESS_FS_TRUNCATE; ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0); if (ruleset_fd == \-1) { -- 2.39.2
