Re: [PATCH] x86/entry_32: Fix segment exceptions

From: Borislav Petkov
Date: Sat Jan 15 2022 - 03:13:29 EST


On Fri, Jan 14, 2022 at 03:48:31PM -0800, Andy Lutomirski wrote:
> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

I actually did this new version:

---
From: Borislav Petkov <bp@xxxxxxx>
Subject: [PATCH] x86/entry_32: Remove GS from the pt_regs offsets and fixup regs

Document how GS (and its stack slot) on 32-bit are used.

Fixes: 9cdbeec40968 ("x86/entry_32: Fix segment exceptions")
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
arch/x86/entry/entry_32.S | 4 +++-
arch/x86/include/asm/extable_fixup_types.h | 5 +++--
arch/x86/lib/insn-eval.c | 5 ++++-
3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index a7ec22b1d06c..addc3966ee20 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -20,7 +20,9 @@
* 1C(%esp) - %ds
* 20(%esp) - %es
* 24(%esp) - %fs
- * 28(%esp) - unused -- was %gs on old stackprotector kernels
+ * 28(%esp) - unused -- was %gs on old stackprotector kernels. %gs is unused in
+ * kernel mode in 32-bit and holds the user value. When handling exceptions, the
+ * C-exception handler address is pushed into the GS-slot on the stack.
* 2C(%esp) - orig_eax
* 30(%esp) - %eip
* 34(%esp) - %cs
diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
index 503622627400..0aa5f4d3234f 100644
--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -20,11 +20,12 @@
#define EX_DATA_FLAG(flag) ((flag) << EX_DATA_FLAG_SHIFT)
#define EX_DATA_IMM(imm) ((imm) << EX_DATA_IMM_SHIFT)

-/* segment regs */
+#ifdef CONFIG_X86_32
+/* segment regs, valid only for 32-bit code, see pt_regoff */
#define EX_REG_DS EX_DATA_REG(8)
#define EX_REG_ES EX_DATA_REG(9)
#define EX_REG_FS EX_DATA_REG(10)
-#define EX_REG_GS EX_DATA_REG(11)
+#endif

/* flags */
#define EX_FLAG_CLEAR_AX EX_DATA_FLAG(1)
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index b781d324211b..cfc4d13b7d5b 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -432,7 +432,10 @@ static const int pt_regoff[] = {
offsetof(struct pt_regs, ds),
offsetof(struct pt_regs, es),
offsetof(struct pt_regs, fs),
- offsetof(struct pt_regs, gs),
+ /*
+ * Can't use that one, see top of entry_32.S
+ * offsetof(struct pt_regs, gs),
+ */
#endif
};

--
2.29.2


--
Regards/Gruss,
Boris.

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