I have only skimmed the diffs. Your knowledge of the deep semantics,
gained by close differential reading of intel and amd docs, is truly
amazing. Many thanks for pushing this through!
I have 2 nits, perhaps stylistic only.
For code like "sp -= 2" or "sp += 2" followed or preceded by a write to the
stack pointer of a uint16_t variable 'x', would it be better/more robust
to rewrite as: "sp -= sizeof(x)" ?
There are a lot of masks constructed using -1. I think it would be clearer
to use 0xffffffff (for 32-bit masks) as that reminds the reader that this
is a bit mask. But it seems that using -1 is how the original code was
written.
On Tue, Jul 9, 2024 at 11:29 PM Paolo Bonzini <pbonz...@redhat.com> wrote:
> This includes bugfixes:
> - allowing IRET from user mode to user mode with SMAP (do not use implicit
> kernel accesses, which break if the stack is in userspace)
>
> - use DPL-level accesses for interrupts and call gates
>
> - various fixes for task switching
>
> And two related cleanups: computing MMU index once for far calls and
> returns
> (including task switches), and using X86Access for TSS access.
>
> Tested with a really ugly patch to kvm-unit-tests, included after
> signature.
>
> Paolo Bonzini (7):
> target/i386/tcg: Allow IRET from user mode to user mode with SMAP
> target/i386/tcg: use PUSHL/PUSHW for error code
> target/i386/tcg: Compute MMU index once
> target/i386/tcg: Use DPL-level accesses for interrupts and call gates
> target/i386/tcg: check for correct busy state before switching to a
> new task
> target/i386/tcg: use X86Access for TSS access
> target/i386/tcg: save current task state before loading new one
>
> Richard Henderson (3):
> target/i386/tcg: Remove SEG_ADDL
> target/i386/tcg: Reorg push/pop within seg_helper.c
> target/i386/tcg: Introduce x86_mmu_index_{kernel_,}pl
>
> target/i386/cpu.h | 11 +-
> target/i386/cpu.c | 27 +-
> target/i386/tcg/seg_helper.c | 606 +++++++++++++++++++----------------
> 3 files changed, 354 insertions(+), 290 deletions(-)
>
> --
> 2.45.2
>
> diff --git a/lib/x86/usermode.c b/lib/x86/usermode.c
> index c3ec0ad7..0bf40c6d 100644
> --- a/lib/x86/usermode.c
> +++ b/lib/x86/usermode.c
> @@ -5,13 +5,15 @@
> #include "x86/desc.h"
> #include "x86/isr.h"
> #include "alloc.h"
> +#include "alloc_page.h"
> #include "setjmp.h"
> #include "usermode.h"
>
> #include "libcflat.h"
> #include <stdint.h>
>
> -#define USERMODE_STACK_SIZE 0x2000
> +#define USERMODE_STACK_ORDER 1 /* 8k */
> +#define USERMODE_STACK_SIZE (1 << (12 + USERMODE_STACK_ORDER))
> #define RET_TO_KERNEL_IRQ 0x20
>
> static jmp_buf jmpbuf;
> @@ -37,9 +39,14 @@ uint64_t run_in_user(usermode_func func, unsigned int
> fault_vector,
> {
> extern char ret_to_kernel;
> volatile uint64_t rax = 0;
> - static unsigned char user_stack[USERMODE_STACK_SIZE];
> + static unsigned char *user_stack;
> handler old_ex;
>
> + if (!user_stack) {
> + user_stack = alloc_pages(USERMODE_STACK_ORDER);
> + printf("%p\n", user_stack);
> + }
> +
> *raised_vector = 0;
> set_idt_entry(RET_TO_KERNEL_IRQ, &ret_to_kernel, 3);
> old_ex = handle_exception(fault_vector,
> @@ -51,6 +58,8 @@ uint64_t run_in_user(usermode_func func, unsigned int
> fault_vector,
> return 0;
> }
>
> + memcpy(user_stack + USERMODE_STACK_SIZE - 8, &func, 8);
> +
> asm volatile (
> /* Prepare kernel SP for exception handlers */
> "mov %%rsp, %[rsp0]\n\t"
> @@ -63,12 +72,13 @@ uint64_t run_in_user(usermode_func func, unsigned int
> fault_vector,
> "pushq %[user_stack_top]\n\t"
> "pushfq\n\t"
> "pushq %[user_cs]\n\t"
> - "lea user_mode(%%rip), %%rax\n\t"
> + "lea user_mode+0x800000(%%rip), %%rax\n\t" //
> smap.flat places usermode addresses at 8MB-16MB
> "pushq %%rax\n\t"
> "iretq\n"
>
> "user_mode:\n\t"
> /* Back up volatile registers before invoking func
> */
> + "pop %%rax\n\t"
> "push %%rcx\n\t"
> "push %%rdx\n\t"
> "push %%rdi\n\t"
> @@ -78,11 +88,12 @@ uint64_t run_in_user(usermode_func func, unsigned int
> fault_vector,
> "push %%r10\n\t"
> "push %%r11\n\t"
> /* Call user mode function */
> + "add $0x800000,%%rbp\n\t"
> "mov %[arg1], %%rdi\n\t"
> "mov %[arg2], %%rsi\n\t"
> "mov %[arg3], %%rdx\n\t"
> "mov %[arg4], %%rcx\n\t"
> - "call *%[func]\n\t"
> + "call *%%rax\n\t"
> /* Restore registers */
> "pop %%r11\n\t"
> "pop %%r10\n\t"
> @@ -112,12 +123,11 @@ uint64_t run_in_user(usermode_func func, unsigned
> int fault_vector,
> [arg2]"m"(arg2),
> [arg3]"m"(arg3),
> [arg4]"m"(arg4),
> - [func]"m"(func),
> [user_ds]"i"(USER_DS),
> [user_cs]"i"(USER_CS),
> [kernel_ds]"rm"(KERNEL_DS),
> [user_stack_top]"r"(user_stack +
> - sizeof(user_stack)),
> + USERMODE_STACK_SIZE - 8),
> [kernel_entry_vector]"i"(RET_TO_KERNEL_IRQ));
>
> handle_exception(fault_vector, old_ex);
> diff --git a/x86/smap.c b/x86/smap.c
> index 9a823a55..65119442 100644
> --- a/x86/smap.c
> +++ b/x86/smap.c
> @@ -2,6 +2,7 @@
> #include <alloc_page.h>
> #include "x86/desc.h"
> #include "x86/processor.h"
> +#include "x86/usermode.h"
> #include "x86/vm.h"
>
> volatile int pf_count = 0;
> @@ -89,6 +90,31 @@ static void check_smap_nowp(void)
> write_cr3(read_cr3());
> }
>
> +#ifdef __x86_64__
> +static void iret(void)
> +{
> + asm volatile(
> + "mov %%rsp, %%rcx;"
> + "movl %%ss, %%ebx; pushq %%rbx; pushq %%rcx;"
> + "pushf;"
> + "movl %%cs, %%ebx; pushq %%rbx; "
> + "lea 1f(%%rip), %%rbx; pushq %%rbx; iretq; 1:"
> +
> + : : : "ebx", "ecx", "cc"); /* RPL=0 */
> +}
> +
> +static void test_user_iret(void)
> +{
> + bool raised_vector;
> + uintptr_t user_iret = (uintptr_t)iret + USER_BASE;
> +
> + run_in_user((usermode_func)user_iret, PF_VECTOR, 0, 0, 0, 0,
> + &raised_vector);
> +
> + report(!raised_vector, "No #PF on CPL=3 DPL=3 iret");
> +}
> +#endif
> +
> int main(int ac, char **av)
> {
> unsigned long i;
> @@ -196,7 +222,9 @@ int main(int ac, char **av)
>
> check_smap_nowp();
>
> - // TODO: implicit kernel access from ring 3 (e.g. int)
> +#ifdef __x86_64__
> + test_user_iret();
> +#endif
>
> return report_summary();
> }
>
>
>
>
