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Peter Anvin" , Andy Lutomirski , Josh Poimboeuf , Peter Zijlstra , Ard Biesheuvel , "Kirill A . Shutemov" , Sohil Mehta , Xin Li , David Woodhouse , Sean Christopherson , Rick Edgecombe , Vegard Nossum , Andrew Cooper , David Laight , Randy Dunlap , Geert Uytterhoeven , Kees Cook , Tony Luck , Alexander Shishkin , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org Subject: [PATCH v10 14/15] x86/traps: Provide additional hints for a kernel stack segment fault Date: Mon, 6 Oct 2025 23:51:18 -0700 Message-ID: <20251007065119.148605-15-sohil.mehta@intel.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251007065119.148605-1-sohil.mehta@intel.com> References: <20251007065119.148605-1-sohil.mehta@intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: "Kirill A. Shutemov" Kernel triggered #SS exceptions are rare, and the faulting instruction may not even have a memory operand. In cases where it does, hints about the cause of the fault can be useful. LASS throws a #GP for any violation except for stack register access, which instead triggers a #SS. Handle a kernel #SS similarly to a #GP and reuse the address decode logic to provide additional hints, such as a non-canonical address or an LASS violation. In case of FRED, before handling #SS as a kernel violation, check if there's a fixup for the exception. Redirect the #SS due to invalid user context on ERETU to userspace. See commit 5105e7687ad3 ("x86/fred: Fixup fault on ERETU by jumping to fred_entrypoint_user") for details. Originally-by: Alexander Shishkin Signed-off-by: Kirill A. Shutemov Signed-off-by: Sohil Mehta --- v10: - Remove the LASS feature check to always provide hints independent of LASS being enabled. - Update printk to use KERN_DEFAULT (checkpatch warning). - Add code comments. --- arch/x86/kernel/traps.c | 43 +++++++++++++++++++++++++++++++++++------ 1 file changed, 37 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 3ee8a36a4e6a..864c614cddab 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -418,12 +418,6 @@ DEFINE_IDTENTRY_ERRORCODE(exc_segment_not_present) SIGBUS, 0, NULL); } =20 -DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment) -{ - do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS, - 0, NULL); -} - DEFINE_IDTENTRY_ERRORCODE(exc_alignment_check) { char *str =3D "alignment check"; @@ -873,6 +867,43 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection) cond_local_irq_disable(regs); } =20 +#define SSFSTR "stack segment fault" + +DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment) +{ + enum kernel_exc_hint hint; + unsigned long exc_addr; + + if (user_mode(regs)) + goto error_trap; + + /* + * With FRED enabled, an invalid user context can lead to an #SS + * trap on ERETU. Fixup the exception and redirect the fault to + * userspace in that case. + */ + if (cpu_feature_enabled(X86_FEATURE_FRED) && + fixup_exception(regs, X86_TRAP_SS, error_code, 0)) + return; + + if (notify_die(DIE_TRAP, SSFSTR, regs, error_code, X86_TRAP_SS, SIGBUS) = =3D=3D NOTIFY_STOP) + return; + + hint =3D get_kernel_exc_address(regs, &exc_addr); + if (hint !=3D EXC_NO_HINT) + printk(KERN_DEFAULT SSFSTR ", %s 0x%lx", kernel_exc_hint_help[hint], exc= _addr); + + /* KASAN only cares about the non-canonical case, clear it otherwise */ + if (hint !=3D EXC_NON_CANONICAL) + exc_addr =3D 0; + + die_addr(SSFSTR, regs, error_code, exc_addr); + return; + +error_trap: + do_error_trap(regs, error_code, SSFSTR, X86_TRAP_SS, SIGBUS, 0, NULL); +} + static bool do_int3(struct pt_regs *regs) { int res; --=20 2.43.0