Date: Thu, 16 Apr 2026 17:46:28 +0000 From: Mark Johnston <markj@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: fa77fecb0e06 - main - pkru.3: Note that the kernel may not respect PKRU protections Message-ID: <69e12074.35df5.2ea31c3d@gitrepo.freebsd.org>
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The branch main has been updated by markj: URL: https://cgit.FreeBSD.org/src/commit/?id=fa77fecb0e06ec697e3d7a9ed899e568f1d2090c commit fa77fecb0e06ec697e3d7a9ed899e568f1d2090c Author: Mark Johnston <markj@FreeBSD.org> AuthorDate: 2026-04-16 17:46:11 +0000 Commit: Mark Johnston <markj@FreeBSD.org> CommitDate: 2026-04-16 17:46:11 +0000 pkru.3: Note that the kernel may not respect PKRU protections There are cases where the kernel will be able to access memory covered by a PKRU key which nomially prohibits accesses. I believe regular copyin()/copyout() are subject to the contents of PKRU, but memory accesses via uiomove_fromphys() will not be. This can arise when performing fault I/O, for instance. I didn't test, but I suspect AIO is another case. Update the man page to acknowledge this. Reviewed by: alc, kib MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D56416 --- lib/libsys/x86/pkru.3 | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/lib/libsys/x86/pkru.3 b/lib/libsys/x86/pkru.3 index 06837a39858d..75580953e6ed 100644 --- a/lib/libsys/x86/pkru.3 +++ b/lib/libsys/x86/pkru.3 @@ -71,13 +71,16 @@ Only one key may apply to a given range at a time. The default protection key index is zero, it is used even if no key was explicitly assigned to the address, or if the key was removed. .Pp -The protection prevents the system from accessing user addresses as well -as the user applications. -When a system call was unable to read or write user memory due to key -protection, it returns the -.Er EFAULT -error code. -Note that some side effects may have occurred if this error is reported. +If the user application attempts a memory access which is prohibited by the +PKRU register, the offending thread receives a synchronous +.Dv SIGSEGV +signal with +.Va si_code +set to +.Dv SEGV_PKUERR . +PKRU protections might prevent the kernel from accessing protected +user addresses when handling system calls, but this is not guaranteed and +must not be relied upon. .Pp Both 64-bit and 32-bit applications can use protection keys. More information about the hardware feature is provided in the IA32 Softwarehome | help
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