sysenter: set kernel stack on enter_usermode

Currently the same stack would be used by all threads, which is a recipe for disaster. Instead, set the kernel stack for sysenter when entering a usermode task. Because ktf does not do context switching, this msr write only happens once. Signed-off-by: Johannes Wikner <[email protected]>
KernelTestFramework · Aug 28, 2023 · 9dbf6c3 · 9dbf6c3
1 parent f3e9d02
commit 9dbf6c3
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Showing 2 changed files with 19 additions and 3 deletions.
diff --git a/arch/x86/entry.S b/arch/x86/entry.S
@@ -60,6 +60,14 @@
     SET_CR3 user_cr3
 .endm
 
+.macro WRITE_MSR msr_reg value
+    mov \msr_reg, %_ASM_CX
+    mov \value, %_ASM_AX
+    mov \value, %_ASM_DX
+    shr $32, %_ASM_DX
+    wrmsr
+.endm
+
 .macro exception_handler sym vec has_error_code
 ENTRY(entry_\sym)
     enter_from_usermode
@@ -173,9 +181,15 @@ ENTRY(enter_usermode)
     PUSHF
 
     /* Save stack pointer onto per-cpu */
-    mov %_ASM_DX, %gs:(usermode_private)
+    mov %_ASM_SP, %gs:usermode_private
 
-    syscall_to_usermode
+    /* provide sysenter the kernel stack to be used */
+    push %_ASM_DX
+    WRITE_MSR $MSR_SYSENTER_ESP, %_ASM_SP
+    pop %_ASM_DX
+
+    /* Move to user stack */
+    mov %_ASM_DX, %_ASM_SP
 
     /* SS + SP */
     push $__USER_DS
@@ -189,6 +203,8 @@ ENTRY(enter_usermode)
     /* CS + IP */
     push $__USER_CS
     push $usermode_stub
+
+    enter_to_usermode
     IRET
 END_FUNC(enter_usermode)
 

diff --git a/common/usermode.c b/common/usermode.c
@@ -99,7 +99,7 @@ static void init_syscall(void) {
 
 static void init_sysenter(void) {
     wrmsr(MSR_SYSENTER_CS, _ul(__KERN_CS));
-    wrmsr(MSR_SYSENTER_ESP, _ul(get_free_page_top(GFP_KERNEL | GFP_USER)));
+    wrmsr(MSR_SYSENTER_ESP, _ul(0)); /* configure the stack on enter_usermode instead */
     wrmsr(MSR_SYSENTER_EIP, _ul(&sysenter_handler_entry));
 }