For mounts that have the new "nosymfollow" option, don't follow symlinks when resolving paths. The new option is similar in spirit to the existing "nodev", "noexec", and "nosuid" options, as well as to the LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD variants have been supporting the "nosymfollow" mount option for a long time with equivalent implementations. Note that symlinks may still be created on file systems mounted with the "nosymfollow" option present. readlink() remains functional, so user space code that is aware of symlinks can still choose to follow them explicitly. Setting the "nosymfollow" mount option helps prevent privileged writers from modifying files unintentionally in case there is an unexpected link along the accessed path. The "nosymfollow" option is thus useful as a defensive measure for systems that need to deal with untrusted file systems in privileged contexts. More information on the history and motivation for this patch can be found here: https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal
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| @@ -16,6 +16,7 @@ · 15 unmodified lines | |||
| 16 | #define MS_REMOUNT 32 /* Alter flags of a mounted FS */ | 16 | #define MS_REMOUNT 32 /* Alter flags of a mounted FS */ |
| 17 | #define MS_MANDLOCK 64 /* Allow mandatory locks on an FS */ | 17 | #define MS_MANDLOCK 64 /* Allow mandatory locks on an FS */ |
| 18 | #define MS_DIRSYNC 128 /* Directory modifications are synchronous */ | 18 | #define MS_DIRSYNC 128 /* Directory modifications are synchronous */ |
| 19 | #define MS_NOSYMFOLLOW 256 /* Do not follow symlinks */ | ||
| 19 | #define MS_NOATIME 1024 /* Do not update access times. */ | 20 | #define MS_NOATIME 1024 /* Do not update access times. */ |
| 20 | #define MS_NODIRATIME 2048 /* Do not update directory access times */ | 21 | #define MS_NODIRATIME 2048 /* Do not update directory access times */ |
| 21 | #define MS_BIND 4096 | 22 | #define MS_BIND 4096 |