Commit-ID: e88221c50cadade0eb4f7f149f4967d760212695 Gitweb: http://git.kernel.org/tip/e88221c50cadade0eb4f7f149f4967d760212695 Author: Ingo Molnar <mingo@xxxxxxxxxx> AuthorDate: Wed, 20 May 2015 11:45:30 +0200 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Wed, 20 May 2015 11:58:26 +0200 x86/fpu: Disable XSAVES* support for now The kernel's handling of 'compacted' xsave state layout is buggy: http://marc.info/?l=linux-kernel&m=142967852317199 I don't have such a system, and the description there is vague, but from extrapolation I guess that there were two kinds of bugs observed: - boot crashes, due to size calculations being wrong and the dynamic allocation allocating a too small xstate area. (This is now fixed in the new FPU code - but still present in stable kernels.) - FPU state corruption and ABI breakage: if signal handlers try to change the FPU state in standard format, which then the kernel tries to restore in the compacted format. These breakages are scary, but they only occur on a small number of systems that have XSAVES* CPU support. Yet we have had XSAVES support in the upstream kernel for a large number of stable kernel releases, and the fixes are involved and unproven. So do the safe resolution first: disable XSAVES* support and only use the standard xstate format. This makes the code work and is easy to backport. On top of this we can work on enabling (and testing!) proper compacted format support, without backporting pressure, on top of the new, cleaned up FPU code. Cc: <stable@xxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- arch/x86/kernel/i387.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c index 00918327..6185d31 100644 --- a/arch/x86/kernel/i387.c +++ b/arch/x86/kernel/i387.c @@ -173,6 +173,21 @@ static void init_thread_xstate(void) xstate_size = sizeof(struct i387_fxsave_struct); else xstate_size = sizeof(struct i387_fsave_struct); + + /* + * Quirk: we don't yet handle the XSAVES* instructions + * correctly, as we don't correctly convert between + * standard and compacted format when interfacing + * with user-space - so disable it for now. + * + * The difference is small: with recent CPUs the + * compacted format is only marginally smaller than + * the standard FPU state format. + * + * ( This is easy to backport while we are fixing + * XSAVES* support. ) + */ + setup_clear_cpu_cap(X86_FEATURE_XSAVES); } /* -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html