[PATCH v2 0/8] x86/sev-es: Fixes for SEV-ES Guest Support

[PATCH v2 0/8] x86/sev-es: Fixes for SEV-ES Guest Support

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

Hi,

here is the second version of my pending SEV-ES fixes. The most
important patches are patch 1 to 5, as they fix warnings and splats
that trigger with various debugging options are enabled.

Patches 6 to 8 fix a correctness issue in the instruction emulation
part of the #VC exception handler.

Please review.

Thanks,

	Joerg

Link to v1: https://lore.kernel.org/lkml/20210512075445.18935-1-joro@8bytes.org/

Changes since v1:

	- Documented why __get_user()/__put_user() are safe to use in
	  the #VC handlers memory access path.

	- Merged the revert into patch 3

	- Refactored code in the instruction decoder and added #GP
	  reporting when getting the instructions linear address fails.

Joerg Roedel (8):
  x86/sev-es: Don't return NULL from sev_es_get_ghcb()
  x86/sev-es: Forward page-faults which happen during emulation
  x86/sev-es: Use __put_user()/__get_user() for data accesses
  x86/sev-es: Fix error message in runtime #VC handler
  x86/sev-es: Leave NMI-mode before sending signals
  x86/insn-eval: Make 0 a valid RIP for insn_get_effective_ip()
  x86/insn: Extend error reporting from
    insn_fetch_from_user[_inatomic]()
  x86/sev-es: Propagate #GP if getting linear instruction address failed

 arch/x86/include/asm/insn-eval.h |   6 +-
 arch/x86/kernel/sev.c            | 127 +++++++++++++++++++++----------
 arch/x86/kernel/umip.c           |  10 +--
 arch/x86/lib/insn-eval.c         |  52 ++++++++-----
 4 files changed, 129 insertions(+), 66 deletions(-)


base-commit: a50c5bebc99c525e7fbc059988c6a5ab8680cb76
-- 
2.31.1

[PATCH v2 1/8] x86/sev-es: Don't return NULL from sev_es_get_ghcb()

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

The sev_es_get_ghcb() is called from several places, but only one of
them checks the return value. The reaction to returning NULL is always
the same: Calling panic() and kill the machine.

Instead of adding checks to all call-places, move the panic() into the
function itself so that it will no longer return NULL.

Fixes: 0786138c78e7 ("x86/sev-es: Add a Runtime #VC Exception Handler")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 4fa111becc93..82bced88153b 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -203,8 +203,18 @@ static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
 	if (unlikely(data->ghcb_active)) {
 		/* GHCB is already in use - save its contents */
 
-		if (unlikely(data->backup_ghcb_active))
-			return NULL;
+		if (unlikely(data->backup_ghcb_active)) {
+			/*
+			 * Backup-GHCB is also already in use. There is no way
+			 * to continue here so just kill the machine. To make
+			 * panic() work, mark GHCBs inactive so that messages
+			 * can be printed out.
+			 */
+			data->ghcb_active        = false;
+			data->backup_ghcb_active = false;
+
+			panic("Unable to handle #VC exception! GHCB and Backup GHCB are already in use");
+		}
 
 		/* Mark backup_ghcb active before writing to it */
 		data->backup_ghcb_active = true;
@@ -1289,7 +1299,6 @@ static __always_inline bool on_vc_fallback_stack(struct pt_regs *regs)
  */
 DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 {
-	struct sev_es_runtime_data *data = this_cpu_read(runtime_data);
 	irqentry_state_t irq_state;
 	struct ghcb_state state;
 	struct es_em_ctxt ctxt;
@@ -1315,16 +1324,6 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 	 */
 
 	ghcb = sev_es_get_ghcb(&state);
-	if (!ghcb) {
-		/*
-		 * Mark GHCBs inactive so that panic() is able to print the
-		 * message.
-		 */
-		data->ghcb_active        = false;
-		data->backup_ghcb_active = false;
-
-		panic("Unable to handle #VC exception! GHCB and Backup GHCB are already in use");
-	}
 
 	vc_ghcb_invalidate(ghcb);
 	result = vc_init_em_ctxt(&ctxt, regs, error_code);
-- 
2.31.1

[PATCH v2 2/8] x86/sev-es: Forward page-faults which happen during emulation

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

When emulating guest instructions for MMIO or IOIO accesses the #VC
handler might get a page-fault and will not be able to complete. Forward
the page-fault in this case to the correct handler instead of killing
the machine.

Fixes: 0786138c78e7 ("x86/sev-es: Add a Runtime #VC Exception Handler")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 82bced88153b..1f428f401bed 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1270,6 +1270,10 @@ static __always_inline void vc_forward_exception(struct es_em_ctxt *ctxt)
 	case X86_TRAP_UD:
 		exc_invalid_op(ctxt->regs);
 		break;
+	case X86_TRAP_PF:
+		write_cr2(ctxt->fi.cr2);
+		exc_page_fault(ctxt->regs, error_code);
+		break;
 	case X86_TRAP_AC:
 		exc_alignment_check(ctxt->regs, error_code);
 		break;
-- 
2.31.1

[PATCH v2 3/8] x86/sev-es: Use __put_user()/__get_user() for data accesses

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

The put_user() and get_user() functions do checks on the address which is
passed to them. They check whether the address is actually a user-space
address and whether its fine to access it. They also call might_fault()
to indicate that they could fault and possibly sleep.

All of these checks are neither wanted nor needed in the #VC exception
handler, which can be invoked from almost any context and also for MMIO
instructions from kernel space on kernel memory. All the #VC handler
wants to know is whether a fault happened when the access was tried.

This is provided by __put_user()/__get_user(), which just do the access
no matter what. Also add comments explaining why __get_user() and
__put_user() are the best choice here and why it is safe to use them
in this context. Also explain why copy_to/from_user can't be used.

In addition, also revert commit

	024f60d6552 x86/sev-es: ("Handle string port IO to kernel memory properly")

because using __get_user()/__put_user() fixes the same problem while
the above commit introduced several problems:

	1) It uses access_ok() which is only allowed in task context.

	2) It uses memcpy() which has no fault handling at all and is
	   thus unsafe to use here.

Fixes: f980f9c31a92 ("x86/sev-es: Compile early handler code into kernel image")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 66 ++++++++++++++++++++++++++++++-------------
 1 file changed, 46 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 1f428f401bed..651b81cd648e 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -315,31 +315,44 @@ static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
 	u16 d2;
 	u8  d1;
 
-	/* If instruction ran in kernel mode and the I/O buffer is in kernel space */
-	if (!user_mode(ctxt->regs) && !access_ok(target, size)) {
-		memcpy(dst, buf, size);
-		return ES_OK;
-	}
-
+	/*
+	 * This function uses __put_user() independent of whether kernel or user
+	 * memory is accessed. This works fine because __put_user() does no
+	 * sanity checks of the pointer being accessed. All that it does is
+	 * to report when the access failed.
+	 *
+	 * Also, this function runs in atomic context, so __put_user() is not
+	 * allowed to sleep. The page-fault handler detects that it is running
+	 * in atomic context and will not try to take mmap_sem and handle the
+	 * fault, so additional pagefault_enable()/disable() calls are not
+	 * needed.
+	 *
+	 * The access can't be done via copy_to_user() here because
+	 * vc_write_mem() must not use string instructions to access unsafe
+	 * memory. The reason is that MOVS is emulated by the #VC handler by
+	 * splitting the move up into a read and a write and taking a nested #VC
+	 * exception on whatever of them is the MMIO access. Using string
+	 * instructions here would cause infinite nesting.
+	 */
 	switch (size) {
 	case 1:
 		memcpy(&d1, buf, 1);
-		if (put_user(d1, target))
+		if (__put_user(d1, target))
 			goto fault;
 		break;
 	case 2:
 		memcpy(&d2, buf, 2);
-		if (put_user(d2, target))
+		if (__put_user(d2, target))
 			goto fault;
 		break;
 	case 4:
 		memcpy(&d4, buf, 4);
-		if (put_user(d4, target))
+		if (__put_user(d4, target))
 			goto fault;
 		break;
 	case 8:
 		memcpy(&d8, buf, 8);
-		if (put_user(d8, target))
+		if (__put_user(d8, target))
 			goto fault;
 		break;
 	default:
@@ -370,30 +383,43 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 	u16 d2;
 	u8  d1;
 
-	/* If instruction ran in kernel mode and the I/O buffer is in kernel space */
-	if (!user_mode(ctxt->regs) && !access_ok(s, size)) {
-		memcpy(buf, src, size);
-		return ES_OK;
-	}
-
+	/*
+	 * This function uses __get_user() independent of whether kernel or user
+	 * memory is accessed. This works fine because __get_user() does no
+	 * sanity checks of the pointer being accessed. All that it does is
+	 * to report when the access failed.
+	 *
+	 * Also, this function runs in atomic context, so __get_user() is not
+	 * allowed to sleep. The page-fault handler detects that it is running
+	 * in atomic context and will not try to take mmap_sem and handle the
+	 * fault, so additional pagefault_enable()/disable() calls are not
+	 * needed.
+	 *
+	 * The access can't be done via copy_from_user() here because
+	 * vc_read_mem() must not use string instructions to access unsafe
+	 * memory. The reason is that MOVS is emulated by the #VC handler by
+	 * splitting the move up into a read and a write and taking a nested #VC
+	 * exception on whatever of them is the MMIO access. Using string
+	 * instructions here would cause infinite nesting.
+	 */
 	switch (size) {
 	case 1:
-		if (get_user(d1, s))
+		if (__get_user(d1, s))
 			goto fault;
 		memcpy(buf, &d1, 1);
 		break;
 	case 2:
-		if (get_user(d2, s))
+		if (__get_user(d2, s))
 			goto fault;
 		memcpy(buf, &d2, 2);
 		break;
 	case 4:
-		if (get_user(d4, s))
+		if (__get_user(d4, s))
 			goto fault;
 		memcpy(buf, &d4, 4);
 		break;
 	case 8:
-		if (get_user(d8, s))
+		if (__get_user(d8, s))
 			goto fault;
 		memcpy(buf, &d8, 8);
 		break;
-- 
2.31.1

[PATCH v2 4/8] x86/sev-es: Fix error message in runtime #VC handler

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

The runtime #VC handler is not "early" anymore. Fix the copy&paste error
and remove that word from the error message.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 651b81cd648e..4fd997bbf059 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1369,7 +1369,7 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 		vc_finish_insn(&ctxt);
 		break;
 	case ES_UNSUPPORTED:
-		pr_err_ratelimited("Unsupported exit-code 0x%02lx in early #VC exception (IP: 0x%lx)\n",
+		pr_err_ratelimited("Unsupported exit-code 0x%02lx in #VC exception (IP: 0x%lx)\n",
 				   error_code, regs->ip);
 		goto fail;
 	case ES_VMM_ERROR:
-- 
2.31.1

[PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

The error path in the runtime #VC handler sends a signal to kill the
current task if the exception was raised from user-space. Some parts of
the #VC handler run in NMI mode, because it is critical that it is not
interrupted (except from an NMI) while the GHCB is in use.

But sending signals in NMI-mode is actually broken and triggers lockdep
warnings. On the other side, when the signal is sent, there is no reason
for the handler to still be in NMI-mode, as the GHCB is not used
anymore.

Leave NMI-mode before entering the error path to get rid of the lockdep
warnings.

Fixes: 62441a1fb532 ("x86/sev-es: Correctly track IRQ states in runtime #VC handler")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 4fd997bbf059..9a64030e74c0 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1343,9 +1343,10 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 		return;
 	}
 
+	instrumentation_begin();
+
 	irq_state = irqentry_nmi_enter(regs);
 	lockdep_assert_irqs_disabled();
-	instrumentation_begin();
 
 	/*
 	 * This is invoked through an interrupt gate, so IRQs are disabled. The
@@ -1395,13 +1396,19 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 		BUG();
 	}
 
-out:
-	instrumentation_end();
 	irqentry_nmi_exit(regs, irq_state);
+	instrumentation_end();
 
 	return;
 
 fail:
+	/*
+	 * Leave NMI mode - the GHCB is not busy anymore and depending on where
+	 * the #VC came from this code is about to either kill the task (when in
+	 * task context) or kill the machine.
+	 */
+	irqentry_nmi_exit(regs, irq_state);
+
 	if (user_mode(regs)) {
 		/*
 		 * Do not kill the machine if user-space triggered the
@@ -1423,7 +1430,9 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
 		panic("Returned from Terminate-Request to Hypervisor\n");
 	}
 
-	goto out;
+	instrumentation_end();
+
+	return;
 }
 
 /* This handler runs on the #VC fall-back stack. It can cause further #VC exceptions */
-- 
2.31.1

[PATCH v2 6/8] x86/insn-eval: Make 0 a valid RIP for insn_get_effective_ip()

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

In theory 0 is a valid value for the instruction pointer, so don't use
it as the error return value from insn_get_effective_ip().

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/lib/insn-eval.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index a67afd74232c..4eecb9c7c6a0 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -1417,7 +1417,7 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 	}
 }
 
-static unsigned long insn_get_effective_ip(struct pt_regs *regs)
+static int insn_get_effective_ip(struct pt_regs *regs, unsigned long *ip)
 {
 	unsigned long seg_base = 0;
 
@@ -1430,10 +1430,12 @@ static unsigned long insn_get_effective_ip(struct pt_regs *regs)
 	if (!user_64bit_mode(regs)) {
 		seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
 		if (seg_base == -1L)
-			return 0;
+			return -EINVAL;
 	}
 
-	return seg_base + regs->ip;
+	*ip = seg_base + regs->ip;
+
+	return 0;
 }
 
 /**
@@ -1455,8 +1457,7 @@ int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
 	unsigned long ip;
 	int not_copied;
 
-	ip = insn_get_effective_ip(regs);
-	if (!ip)
+	if (insn_get_effective_ip(regs, &ip))
 		return 0;
 
 	not_copied = copy_from_user(buf, (void __user *)ip, MAX_INSN_SIZE);
@@ -1484,8 +1485,7 @@ int insn_fetch_from_user_inatomic(struct pt_regs *regs, unsigned char buf[MAX_IN
 	unsigned long ip;
 	int not_copied;
 
-	ip = insn_get_effective_ip(regs);
-	if (!ip)
+	if (insn_get_effective_ip(regs, &ip))
 		return 0;
 
 	not_copied = __copy_from_user_inatomic(buf, (void __user *)ip, MAX_INSN_SIZE);
-- 
2.31.1

[PATCH v2 7/8] x86/insn: Extend error reporting from insn_fetch_from_user[_inatomic]()

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

The error reporting from the insn_fetch_from_user*() functions is not
very verbose. Extend it to include information on whether the linear
RIP could not be calculated or whether the memory access faulted.

This will be used in the SEV-ES code to propagate the correct
exception depending on what went wrong during instruction fetch.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/include/asm/insn-eval.h |  6 +++--
 arch/x86/kernel/sev.c            |  8 +++----
 arch/x86/kernel/umip.c           | 10 ++++-----
 arch/x86/lib/insn-eval.c         | 38 +++++++++++++++++++++++---------
 4 files changed, 41 insertions(+), 21 deletions(-)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 91d7182ad2d6..8362f1ce4b00 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -22,9 +22,11 @@ int insn_get_modrm_reg_off(struct insn *insn, struct pt_regs *regs);
 unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx);
 int insn_get_code_seg_params(struct pt_regs *regs);
 int insn_fetch_from_user(struct pt_regs *regs,
-			 unsigned char buf[MAX_INSN_SIZE]);
+			 unsigned char buf[MAX_INSN_SIZE],
+			 int *copied);
 int insn_fetch_from_user_inatomic(struct pt_regs *regs,
-				  unsigned char buf[MAX_INSN_SIZE]);
+				  unsigned char buf[MAX_INSN_SIZE],
+				  int *copied);
 bool insn_decode_from_regs(struct insn *insn, struct pt_regs *regs,
 			   unsigned char buf[MAX_INSN_SIZE], int buf_size);
 
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 9a64030e74c0..1edb6cd5e308 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -258,17 +258,17 @@ static int vc_fetch_insn_kernel(struct es_em_ctxt *ctxt,
 static enum es_result __vc_decode_user_insn(struct es_em_ctxt *ctxt)
 {
 	char buffer[MAX_INSN_SIZE];
-	int res;
+	int insn_bytes = 0, res;
 
-	res = insn_fetch_from_user_inatomic(ctxt->regs, buffer);
-	if (!res) {
+	res = insn_fetch_from_user_inatomic(ctxt->regs, buffer, &insn_bytes);
+	if (res) {
 		ctxt->fi.vector     = X86_TRAP_PF;
 		ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;
 		ctxt->fi.cr2        = ctxt->regs->ip;
 		return ES_EXCEPTION;
 	}
 
-	if (!insn_decode_from_regs(&ctxt->insn, ctxt->regs, buffer, res))
+	if (!insn_decode_from_regs(&ctxt->insn, ctxt->regs, buffer, insn_bytes))
 		return ES_DECODE_FAILED;
 
 	if (ctxt->insn.immediate.got)
diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 8daa70b0d2da..b005ef42682e 100644
--- a/arch/x86/kernel/umip.c
+++ b/arch/x86/kernel/umip.c
@@ -346,13 +346,13 @@ bool fixup_umip_exception(struct pt_regs *regs)
 	if (!regs)
 		return false;
 
-	nr_copied = insn_fetch_from_user(regs, buf);
-
 	/*
-	 * The insn_fetch_from_user above could have failed if user code
-	 * is protected by a memory protection key. Give up on emulation
-	 * in such a case.  Should we issue a page fault?
+	 * Give up on emulation if fetching the instruction failed. Should we
+	 * issue a page fault or a #GP?
 	 */
+	if (!insn_fetch_from_user(regs, buf, NULL))
+		return false;
+
 	if (!nr_copied)
 		return false;
 
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 4eecb9c7c6a0..d8a057ba0895 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -1442,27 +1442,36 @@ static int insn_get_effective_ip(struct pt_regs *regs, unsigned long *ip)
  * insn_fetch_from_user() - Copy instruction bytes from user-space memory
  * @regs:	Structure with register values as seen when entering kernel mode
  * @buf:	Array to store the fetched instruction
+ * @copied:	Pointer to an int where the number of copied instruction bytes
+ *		is stored. Can be NULL.
  *
  * Gets the linear address of the instruction and copies the instruction bytes
  * to the buf.
  *
  * Returns:
  *
- * Number of instruction bytes copied.
+ * -EINVAL if the linear address of the instruction could not be calculated
+ * -EFAULT if nothing was copied
+ *       0 on success
  *
- * 0 if nothing was copied.
  */
-int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
+int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE],
+			 int *copied)
 {
 	unsigned long ip;
 	int not_copied;
+	int bytes;
 
 	if (insn_get_effective_ip(regs, &ip))
-		return 0;
+		return -EINVAL;
 
 	not_copied = copy_from_user(buf, (void __user *)ip, MAX_INSN_SIZE);
 
-	return MAX_INSN_SIZE - not_copied;
+	bytes = MAX_INSN_SIZE - not_copied;
+	if (copied)
+		*copied = bytes;
+
+	return bytes ? 0 : -EFAULT;
 }
 
 /**
@@ -1470,27 +1479,36 @@ int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
  *                                   while in atomic code
  * @regs:	Structure with register values as seen when entering kernel mode
  * @buf:	Array to store the fetched instruction
+ * @copied:	Pointer to an int where the number of copied instruction bytes
+ *		is stored. Can be NULL.
  *
  * Gets the linear address of the instruction and copies the instruction bytes
  * to the buf. This function must be used in atomic context.
  *
  * Returns:
  *
- * Number of instruction bytes copied.
+ * -EINVAL if the linear address of the instruction could not be calculated
+ * -EFAULT if nothing was copied
+ *       0 on success
  *
- * 0 if nothing was copied.
  */
-int insn_fetch_from_user_inatomic(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
+int insn_fetch_from_user_inatomic(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE],
+				  int *copied)
 {
 	unsigned long ip;
 	int not_copied;
+	int bytes;
 
 	if (insn_get_effective_ip(regs, &ip))
-		return 0;
+		return -EINVAL;
 
 	not_copied = __copy_from_user_inatomic(buf, (void __user *)ip, MAX_INSN_SIZE);
 
-	return MAX_INSN_SIZE - not_copied;
+	bytes = MAX_INSN_SIZE - not_copied;
+	if (copied)
+		*copied = bytes;
+
+	return bytes ? 0 : -EFAULT;
 }
 
 /**
-- 
2.31.1

[PATCH v2 8/8] x86/sev-es: Propagate #GP if getting linear instruction address failed

[Joerg Roedel]

From: Joerg Roedel <jroedel@suse.de>

When an instruction is fetched from user-space, segmentation needs to
be taken into account. This means that getting the linear address of
an instruction can fail. Hardware would raise a #GP
exception in that case, but the #VC exception handler would emulate it
as a page-fault.

The insn_fetch_from_user*() functions now provide the relevant
information in case of an failure. Use that and propagate a #GP when
the linear address of an instruction to fetch could not be calculated.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 1edb6cd5e308..4736290361e4 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -261,11 +261,16 @@ static enum es_result __vc_decode_user_insn(struct es_em_ctxt *ctxt)
 	int insn_bytes = 0, res;
 
 	res = insn_fetch_from_user_inatomic(ctxt->regs, buffer, &insn_bytes);
-	if (res) {
+	if (res == -EFAULT) {
 		ctxt->fi.vector     = X86_TRAP_PF;
 		ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;
 		ctxt->fi.cr2        = ctxt->regs->ip;
 		return ES_EXCEPTION;
+	} else if (res == -EINVAL) {
+		ctxt->fi.vector     = X86_TRAP_GP;
+		ctxt->fi.error_code = 0;
+		ctxt->fi.cr2        = 0;
+		return ES_EXCEPTION;
 	}
 
 	if (!insn_decode_from_regs(&ctxt->insn, ctxt->regs, buffer, insn_bytes))
-- 
2.31.1

Re: [PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

[Peter Zijlstra]

On Wed, May 19, 2021 at 03:52:48PM +0200, Joerg Roedel wrote:
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1343,9 +1343,10 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		return;
>  	}
>  
> +	instrumentation_begin();
> +
>  	irq_state = irqentry_nmi_enter(regs);
>  	lockdep_assert_irqs_disabled();
> -	instrumentation_begin();
>  
>  	/*
>  	 * This is invoked through an interrupt gate, so IRQs are disabled. The

That's just plain wrong. No instrumentation is allowed before you enter
the exception context.

> @@ -1395,13 +1396,19 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		BUG();
>  	}
>  
> -out:
> -	instrumentation_end();
>  	irqentry_nmi_exit(regs, irq_state);
> +	instrumentation_end();

And this can't be right either, same issue, no instrumentation is
allowed after you leave the exception context.

>  
>  	return;
>  
>  fail:
> +	/*
> +	 * Leave NMI mode - the GHCB is not busy anymore and depending on where
> +	 * the #VC came from this code is about to either kill the task (when in
> +	 * task context) or kill the machine.
> +	 */
> +	irqentry_nmi_exit(regs, irq_state);
> +

And this is wrong too; because at this point the handler doesn't run in
_any_ context anymore, certainly not one you can call regular C code
from.

>  	if (user_mode(regs)) {
>  		/*
>  		 * Do not kill the machine if user-space triggered the
> @@ -1423,7 +1430,9 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		panic("Returned from Terminate-Request to Hypervisor\n");
>  	}
>  
> -	goto out;
> +	instrumentation_end();
> +
> +	return;
>  }


You either get to do what MCE does, or what MCE does. That is, either
use task_work or MCE_USER and have the _user() handler use
irqentry_enter_from_user_mode().

The above is an absolute no-go.

Re: [PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

[Joerg Roedel]

Hi Peter,

thanks for your review.

On Wed, May 19, 2021 at 07:54:50PM +0200, Peter Zijlstra wrote:
> On Wed, May 19, 2021 at 03:52:48PM +0200, Joerg Roedel wrote:
> > --- a/arch/x86/kernel/sev.c
> > +++ b/arch/x86/kernel/sev.c
> > @@ -1343,9 +1343,10 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
> >  		return;
> >  	}
> >  
> > +	instrumentation_begin();
> > +
> >  	irq_state = irqentry_nmi_enter(regs);
> >  	lockdep_assert_irqs_disabled();
> > -	instrumentation_begin();
> >  
> >  	/*
> >  	 * This is invoked through an interrupt gate, so IRQs are disabled. The
> 
> That's just plain wrong. No instrumentation is allowed before you enter
> the exception context.

Okay.

> > +	irqentry_nmi_exit(regs, irq_state);
> > +
> 
> And this is wrong too; because at this point the handler doesn't run in
> _any_ context anymore, certainly not one you can call regular C code
> from.

The #VC handler is at this point not running on the IST stack anymore,
but on the stack it came from or on the task stack. So my believe was
that at this point it inherits the context it came from (just like the
page-fault handler). But I also don't fully understand the context
tracking, so is my assumption wrong?

Regards,

	Joerg

Re: [PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

[Peter Zijlstra]

On Wed, May 19, 2021 at 09:13:08PM +0200, Joerg Roedel wrote:
> Hi Peter,
> 
> thanks for your review.
> 
> On Wed, May 19, 2021 at 07:54:50PM +0200, Peter Zijlstra wrote:
> > On Wed, May 19, 2021 at 03:52:48PM +0200, Joerg Roedel wrote:
> > > --- a/arch/x86/kernel/sev.c
> > > +++ b/arch/x86/kernel/sev.c
> > > @@ -1343,9 +1343,10 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
> > >  		return;
> > >  	}
> > >  
> > > +	instrumentation_begin();
> > > +
> > >  	irq_state = irqentry_nmi_enter(regs);
> > >  	lockdep_assert_irqs_disabled();
> > > -	instrumentation_begin();
> > >  
> > >  	/*
> > >  	 * This is invoked through an interrupt gate, so IRQs are disabled. The
> > 
> > That's just plain wrong. No instrumentation is allowed before you enter
> > the exception context.
> 
> Okay.
> 
> > > +	irqentry_nmi_exit(regs, irq_state);
> > > +
> > 
> > And this is wrong too; because at this point the handler doesn't run in
> > _any_ context anymore, certainly not one you can call regular C code
> > from.
> 
> The #VC handler is at this point not running on the IST stack anymore,
> but on the stack it came from or on the task stack. So my believe was
> that at this point it inherits the context it came from (just like the
> page-fault handler). But I also don't fully understand the context
> tracking, so is my assumption wrong?

Being on the right stack is only part of the issue; you also need to
make sure your runtime environment is set up.

Regular kernel C expects a whole lot of things to be present; esp. so
with all the debug options on. The irqentry_*_enter() family of
functions very carefully sets up this environment and the
irqentry_*_exit() undoes it again. Before and after you really cannot
run normal code.

Just an example, RCU might not be watching, it might think the CPU is in
userspace and advance the GP while you're relying on it not doing so.

Similarly lockdep is in some undefined state and any lock used can
trigger random 'funny' things.

Just because this is 'C', doesn't immediately mean you can go call any
random function. Up until recently most of this was in ASM. There's a
reason for the noinstr annotations.

Re: [PATCH v2 7/8] x86/insn: Extend error reporting from insn_fetch_from_user[_inatomic]()

[Borislav Petkov]

On Wed, May 19, 2021 at 03:52:50PM +0200, Joerg Roedel wrote:
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 4eecb9c7c6a0..d8a057ba0895 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -1442,27 +1442,36 @@ static int insn_get_effective_ip(struct pt_regs *regs, unsigned long *ip)
>   * insn_fetch_from_user() - Copy instruction bytes from user-space memory
>   * @regs:	Structure with register values as seen when entering kernel mode
>   * @buf:	Array to store the fetched instruction
> + * @copied:	Pointer to an int where the number of copied instruction bytes
> + *		is stored. Can be NULL.
>   *
>   * Gets the linear address of the instruction and copies the instruction bytes
>   * to the buf.
>   *
>   * Returns:
>   *
> - * Number of instruction bytes copied.
> + * -EINVAL if the linear address of the instruction could not be calculated
> + * -EFAULT if nothing was copied
> + *       0 on success
>   *
> - * 0 if nothing was copied.
>   */
> -int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
> +int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE],
> +			 int *copied)
>  {
>  	unsigned long ip;
>  	int not_copied;
> +	int bytes;
>  
>  	if (insn_get_effective_ip(regs, &ip))
> -		return 0;
> +		return -EINVAL;
>  
>  	not_copied = copy_from_user(buf, (void __user *)ip, MAX_INSN_SIZE);
>  
> -	return MAX_INSN_SIZE - not_copied;
> +	bytes = MAX_INSN_SIZE - not_copied;
> +	if (copied)
> +		*copied = bytes;
> +
> +	return bytes ? 0 : -EFAULT;

Why not simpler?

return value >= 0 says how many bytes were copied
return value < 0 means some kind of error

And then you don't need @copied...

Ditto for the other one.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette