From ef5dd6a0c828b6fbd9d595e5772fcb51ff86697e Mon Sep 17 00:00:00 2001 From: Catalin Marinas Date: Wed, 28 Oct 2020 14:55:24 +0000 Subject: arm64: mte: Document that user PSTATE.TCO is ignored by kernel uaccess On exception entry, the kernel explicitly resets the PSTATE.TCO (tag check override) so that any kernel memory accesses will be checked (the bit is restored on exception return). This has the side-effect that the uaccess routines will not honour the PSTATE.TCO that may have been set by the user prior to a syscall. There is no issue in practice since PSTATE.TCO is expected to be used only for brief periods in specific routines (e.g. garbage collection). To control the tag checking mode of the uaccess routines, the user will have to invoke a corresponding prctl() call. Document the kernel behaviour w.r.t. PSTATE.TCO accordingly. Signed-off-by: Catalin Marinas Fixes: df9d7a22dd21 ("arm64: mte: Add Memory Tagging Extension documentation") Reviewed-by: Vincenzo Frascino Cc: Will Deacon Cc: Szabolcs Nagy Signed-off-by: Will Deacon --- Documentation/arm64/memory-tagging-extension.rst | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'Documentation/arm64') diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst index 034d37c605e8..b540178a93f8 100644 --- a/Documentation/arm64/memory-tagging-extension.rst +++ b/Documentation/arm64/memory-tagging-extension.rst @@ -102,7 +102,9 @@ applications. system call) are not checked if the user thread tag checking mode is ``PR_MTE_TCF_NONE`` or ``PR_MTE_TCF_ASYNC``. If the tag checking mode is ``PR_MTE_TCF_SYNC``, the kernel makes a best effort to check its user -address accesses, however it cannot always guarantee it. +address accesses, however it cannot always guarantee it. Kernel accesses +to user addresses are always performed with an effective ``PSTATE.TCO`` +value of zero, regardless of the user configuration. Excluding Tags in the ``IRG``, ``ADDG`` and ``SUBG`` instructions ----------------------------------------------------------------- -- cgit v1.2.3-59-g8ed1b