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ptrace: Fix ->ptracer_cred handling for PTRACE_TRACEME
Fix two issues: When called for PTRACE_TRACEME, ptrace_link() would obtain an RCU reference to the parent's objective credentials, then give that pointer to get_cred(). However, the object lifetime rules for things like struct cred do not permit unconditionally turning an RCU reference into a stable reference. PTRACE_TRACEME records the parent's credentials as if the parent was acting as the subject, but that's not the case. If a malicious unprivileged child uses PTRACE_TRACEME and the parent is privileged, and at a later point, the parent process becomes attacker-controlled (because it drops privileges and calls execve()), the attacker ends up with control over two processes with a privileged ptrace relationship, which can be abused to ptrace a suid binary and obtain root privileges. Fix both of these by always recording the credentials of the process that is requesting the creation of the ptrace relationship: current_cred() can't change under us, and current is the proper subject for access control. This change is theoretically userspace-visible, but I am not aware of any code that it will actually break. Fixes: 64b875f ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP") Signed-off-by: Jann Horn <email@example.com> Acked-by: Oleg Nesterov <firstname.lastname@example.org> Cc: email@example.com Signed-off-by: Linus Torvalds <firstname.lastname@example.org>
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