On 18.07.18 11:40, Joerg Roedel wrote:
From: Joerg Roedel <jroedel@xxxxxxx> It can happen that we enter the kernel from kernel-mode and on the entry-stack. The most common way this happens is when we get an exception while loading the user-space segment registers on the kernel-to-userspace exit path. The segment loading needs to be done after the entry-stack switch, because the stack-switch needs kernel %fs for per_cpu access. When this happens, we need to make sure that we leave the kernel with the entry-stack again, so that the interrupted code-path runs on the right stack when switching to the user-cr3. We do this by detecting this condition on kernel-entry by checking CS.RPL and %esp, and if it happens, we copy over the complete content of the entry stack to the task-stack. This needs to be done because once we enter the exception handlers we might be scheduled out or even migrated to a different CPU, so that we can't rely on the entry-stack contents. We also leave a marker in the stack-frame to detect this condition on the exit path. On the exit path the copy is reversed, we copy all of the remaining task-stack back to the entry-stack and switch to it. Signed-off-by: Joerg Roedel <jroedel@xxxxxxx> --- arch/x86/entry/entry_32.S | 116 +++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 115 insertions(+), 1 deletion(-) diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index 7635925..9d6eceb 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -294,6 +294,9 @@ * copied there. So allocate the stack-frame on the task-stack and * switch to it before we do any copying. */ + +#define CS_FROM_ENTRY_STACK (1 << 31) + .macro SWITCH_TO_KERNEL_STACKALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV@@ -316,6 +319,16 @@ /* Load top of task-stack into %edi */ movl TSS_entry2task_stack(%edi), %edi+ /*+ * Clear unused upper bits of the dword containing the word-sized CS + * slot in pt_regs in case hardware didn't clear it for us. + */ + andl $(0x0000ffff), PT_CS(%esp) + + /* Special case - entry from kernel mode via entry stack */ + testl $SEGMENT_RPL_MASK, PT_CS(%esp) + jz .Lentry_from_kernel_\@ + /* Bytes to copy */ movl $PTREGS_SIZE, %ecx@@ -329,8 +342,8 @@*/ addl $(4 * 4), %ecx-.Lcopy_pt_regs_\@:#endif +.Lcopy_pt_regs_\@:/* Allocate frame on task-stack */subl %ecx, %edi @@ -346,6 +359,56 @@ cld rep movsl+ jmp .Lend_\@+ +.Lentry_from_kernel_\@: + + /* + * This handles the case when we enter the kernel from + * kernel-mode and %esp points to the entry-stack. When this + * happens we need to switch to the task-stack to run C code, + * but switch back to the entry-stack again when we approach + * iret and return to the interrupted code-path. This usually + * happens when we hit an exception while restoring user-space + * segment registers on the way back to user-space. + * + * When we switch to the task-stack here, we can't trust the + * contents of the entry-stack anymore, as the exception handler + * might be scheduled out or moved to another CPU. Therefore we + * copy the complete entry-stack to the task-stack and set a + * marker in the iret-frame (bit 31 of the CS dword) to detect + * what we've done on the iret path. + * + * On the iret path we copy everything back and switch to the + * entry-stack, so that the interrupted kernel code-path + * continues on the same stack it was interrupted with. + * + * Be aware that an NMI can happen anytime in this code. + * + * %esi: Entry-Stack pointer (same as %esp) + * %edi: Top of the task stack + */ + + /* Calculate number of bytes on the entry stack in %ecx */ + movl %esi, %ecx + + /* %ecx to the top of entry-stack */ + andl $(MASK_entry_stack), %ecx + addl $(SIZEOF_entry_stack), %ecx + + /* Number of bytes on the entry stack to %ecx */ + sub %esi, %ecx + + /* Mark stackframe as coming from entry stack */ + orl $CS_FROM_ENTRY_STACK, PT_CS(%esp) + + /* + * %esi and %edi are unchanged, %ecx contains the number of + * bytes to copy. The code at .Lcopy_pt_regs_\@ will allocate + * the stack-frame on task-stack and copy everything over + */ + jmp .Lcopy_pt_regs_\@ + .Lend_\@: .endm@@ -404,6 +467,56 @@.endm/*+ * This macro handles the case when we return to kernel-mode on the iret + * path and have to switch back to the entry stack. + * + * See the comments below the .Lentry_from_kernel_\@ label in the + * SWITCH_TO_KERNEL_STACK macro for more details. + */ +.macro PARANOID_EXIT_TO_KERNEL_MODE + + /* + * Test if we entered the kernel with the entry-stack. Most + * likely we did not, because this code only runs on the + * return-to-kernel path. + */ + testl $CS_FROM_ENTRY_STACK, PT_CS(%esp) + jz .Lend_\@ + + /* Unlikely slow-path */ + + /* Clear marker from stack-frame */ + andl $(~CS_FROM_ENTRY_STACK), PT_CS(%esp) + + /* Copy the remaining task-stack contents to entry-stack */ + movl %esp, %esi + movl PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi + + /* Bytes on the task-stack to ecx */ + movl PER_CPU_VAR(cpu_tss_rw + TSS_sp1), %ecx + subl %esi, %ecx + + /* Allocate stack-frame on entry-stack */ + subl %ecx, %edi + + /* + * Save future stack-pointer, we must not switch until the + * copy is done, otherwise the NMI handler could destroy the + * contents of the task-stack we are about to copy. + */ + movl %edi, %ebx + + /* Do the copy */ + shrl $2, %ecx + cld + rep movsl + + /* Safe to switch to entry-stack now */ + movl %ebx, %esp + +.Lend_\@: +.endm +/* * %eax: prev task * %edx: next task */ @@ -764,6 +877,7 @@ restore_all:restore_all_kernel:TRACE_IRQS_IRET + PARANOID_EXIT_TO_KERNEL_MODE RESTORE_REGS 4 jmp .Lirq_return
I've bisected down a boot breakage on Intel Quark board (config attached) to this commit (b92a165df17e, I additionally had to apply d1b47a7c9efc). The kernel prints out nothing if this is in.
The board is an Siemens IOT2000, I will check if this can also be triggered on a similar Galileo Gen2. Qemu does not like to reproduce it, unfortunately.
The commit look unsuspicious at first glance - maybe it is just changing some layout in an unfortunate way. Any ideas?
Thanks, Jan -- Siemens AG, Corporate Technology, CT RDA IOT SES-DE Corporate Competence Center Embedded Linux
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