path: root/Documentation/x86
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authorThomas Gleixner <tglx@linutronix.de>2019-04-14 15:59:57 (GMT)
committerBorislav Petkov <bp@suse.de>2019-04-17 13:14:28 (GMT)
commit2a594d4ccf3f10f80b77d71bd3dad10813ac0137 (patch)
tree9cd3da8abe070c47436bf0217a0cb8ee9720f7ad /Documentation/x86
parent1bdb67e5aa2d5d43c48cb7d93393fcba276c9e71 (diff)
x86/exceptions: Split debug IST stack
The debug IST stack is actually two separate debug stacks to handle #DB recursion. This is required because the CPU starts always at top of stack on exception entry, which means on #DB recursion the second #DB would overwrite the stack of the first. The low level entry code therefore adjusts the top of stack on entry so a secondary #DB starts from a different stack page. But the stack pages are adjacent without a guard page between them. Split the debug stack into 3 stacks which are separated by guard pages. The 3rd stack is never mapped into the cpu_entry_area and is only there to catch triple #DB nesting: --- top of DB_stack <- Initial stack --- end of DB_stack guard page --- top of DB1_stack <- Top of stack after entering first #DB --- end of DB1_stack guard page --- top of DB2_stack <- Top of stack after entering second #DB --- end of DB2_stack guard page If DB2 would not act as the final guard hole, a second #DB would point the top of #DB stack to the stack below #DB1 which would be valid and not catch the not so desired triple nesting. The backing store does not allocate any memory for DB2 and its guard page as it is not going to be mapped into the cpu_entry_area. - Adjust the low level entry code so it adjusts top of #DB with the offset between the stacks instead of exception stack size. - Make the dumpstack code aware of the new stacks. - Adjust the in_debug_stack() implementation and move it into the NMI code where it belongs. As this is NMI hotpath code, it just checks the full area between top of DB_stack and bottom of DB1_stack without checking for the guard page. That's correct because the NMI cannot hit a stackpointer pointing to the guard page between DB and DB1 stack. Even if it would, then the NMI operation still is unaffected, but the resume of the debug exception on the topmost DB stack will crash by touching the guard page. [ bp: Make exception_stack_names static const char * const ] Suggested-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Baoquan He <bhe@redhat.com> Cc: "Chang S. Bae" <chang.seok.bae@intel.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Dominik Brodowski <linux@dominikbrodowski.net> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Joerg Roedel <jroedel@suse.de> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: linux-doc@vger.kernel.org Cc: Masahiro Yamada <yamada.masahiro@socionext.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Qian Cai <cai@lca.pw> Cc: Sean Christopherson <sean.j.christopherson@intel.com> Cc: x86-ml <x86@kernel.org> Link: https://lkml.kernel.org/r/20190414160145.439944544@linutronix.de
Diffstat (limited to 'Documentation/x86')
1 files changed, 6 insertions, 1 deletions
diff --git a/Documentation/x86/kernel-stacks b/Documentation/x86/kernel-stacks
index 1b04596..d1bfb0b 100644
--- a/Documentation/x86/kernel-stacks
+++ b/Documentation/x86/kernel-stacks
@@ -76,7 +76,7 @@ The currently assigned IST stacks are :-
middle of switching stacks. Using IST for NMI events avoids making
assumptions about the previous state of the kernel stack.
Used for hardware debug interrupts (interrupt 1) and for software
debug interrupts (INT3).
@@ -86,6 +86,11 @@ The currently assigned IST stacks are :-
avoids making assumptions about the previous state of the kernel
+ To handle nested #DB correctly there exist two instances of DB stacks. On
+ #DB entry the IST stackpointer for #DB is switched to the second instance
+ so a nested #DB starts from a clean stack. The nested #DB switches
+ the IST stackpointer to a guard hole to catch triple nesting.
Used for interrupt 18 - Machine Check Exception (#MC).