| Message ID | 1531906876-13451-11-git-send-email-joro@8bytes.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 18.07.18 11:40, Joerg Roedel wrote: > From: Joerg Roedel <jroedel@suse.de> > > 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@suse.de> > --- > 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_STACK > > ALTERNATIVE "", "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
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_STACK ALTERNATIVE "", "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