[PATCH 0/2] x86/fpu/xstate: Follow up on the init_fpstate fallout again

[PATCH 0/2] x86/fpu/xstate: Follow up on the init_fpstate fallout again

[Chang S. Bae]

Dear maintainers,

This series is following up on the last fix [2]. I thought I could
forget about it with that. But, I was wrong because now this was
realized as an incomplete solution -- my bad. Here is some context for
this series:

The last fix [3] has resolved the case when copying the initialized
dynamic state from init_fpstate to the user buffer in
__copy_xstate_to_uabi_buf(). (This was intended to resolve the fallout
of the init_fpstate fix [1].)

But, when copying the *non-initialized* dynamic state from the task
xstate, the code [4] unconditionally retrieves the address in
init_fpstate which is needless. Consequently, this triggers a
false-positive warning as shown in [5] which meaninglessly confuses
users.

With these repetitive surgeries, a more solid and comprehensive
solution is more helpful I thought. Considerably removing init_fpstate
from this loop is not impossible here because dynamic states have an
all-zeros init state. Then, zeroing the user buffer instead of
retrieving init_fpstate resolves the issue and simplifies the code.

These issues were discovered from the KVM execution with launching a
guest and running the KVM self-test as __copy_xstate_to_uabi_buf() was
called. But, the negligibly missing ptrace test could have disclosed
them too. So that case is included here.

Thanks,
Chang

[1] https://lore.kernel.org/lkml/20220824191223.1248-1-chang.seok.bae@intel.com/
[2] https://lore.kernel.org/lkml/20221018221349.4196-1-chang.seok.bae@intel.com/
[3] https://lore.kernel.org/lkml/20221021185844.13472-1-chang.seok.bae@intel.com/
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n1156
[5] https://lore.kernel.org/kvm/20230221163655.920289-2-mizhang@google.com/

Chang S. Bae (2):
  x86/fpu/xstate: Prevent false-positive warning in
    __copy_xstate_uabi_buf()
  selftests/x86/amx: Add a ptrace test

 arch/x86/kernel/fpu/xstate.c      |  30 ++++-----
 tools/testing/selftests/x86/amx.c | 108 +++++++++++++++++++++++++++++-
 2 files changed, 119 insertions(+), 19 deletions(-)


base-commit: 7fa08de735e41001a70c8ca869b2b159d74c2339
-- 
2.17.1

[PATCH 1/2] x86/fpu/xstate: Prevent false-positive warning in __copy_xstate_uabi_buf()

[Chang S. Bae]

__copy_xstate_to_uabi_buf() copies either from the tasks XSAVE buffer
or from init_fpstate into the ptrace buffer. Dynamic features, like
XTILEDATA, have an all zeroes init state and are not saved in
init_fpstate, which means the corresponding bit is not set in the
xfeatures bitmap of the init_fpstate header.

But __copy_xstate_to_uabi_buf() retrieves addresses for both the tasks
xstate and init_fpstate unconditionally via __raw_xsave_addr().

So if the tasks XSAVE buffer has a dynamic feature set, then the
address retrieval for init_fpstate triggers the warning in
__raw_xsave_addr() which checks the feature bit in the init_fpstate
header.

Remove the address retrieval from init_fpstate for extended features.
They have an all zeroes init state so init_fpstate has zeros for them.
Then zeroing the user buffer for the init state is the same as copying
them from init_fpstate.

Fixes: 2308ee57d93d ("x86/fpu/amx: Enable the AMX feature in 64-bit mode")
Reported-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Tested-by: Mingwei Zhang <mizhang@google.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/kvm/20230221163655.920289-2-mizhang@google.com/
---
Thanks, Mingwei for detecting the issue and Thomas for explaining the
problem with the well-written description. The background and problem
sections in this changelog came from his write-up.

I acknowledge that Mingwei's patch (in the link above) can solve the
problem. But, I would rather propose this approach as it simplifies
the code which is considered good in this sensitive copy function.

This mostly zeroed init_fpstate is still useful, e.g., when copying
the init state between buffers with the same format -- like in
fpu_clone(). But, this case between different XSAVE formats looks a
bit different than that.
---
 arch/x86/kernel/fpu/xstate.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 714166cc25f2..0bab497c9436 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1118,21 +1118,20 @@ void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
 	zerofrom = offsetof(struct xregs_state, extended_state_area);
 
 	/*
-	 * The ptrace buffer is in non-compacted XSAVE format.  In
-	 * non-compacted format disabled features still occupy state space,
-	 * but there is no state to copy from in the compacted
-	 * init_fpstate. The gap tracking will zero these states.
-	 */
-	mask = fpstate->user_xfeatures;
-
-	/*
-	 * Dynamic features are not present in init_fpstate. When they are
-	 * in an all zeros init state, remove those from 'mask' to zero
-	 * those features in the user buffer instead of retrieving them
-	 * from init_fpstate.
+	 * This 'mask' indicates which states to copy from fpstate.
+	 * Those extended states that are not present in fpstate are
+	 * either disabled or initialized:
+	 *
+	 * In non-compacted format, disabled features still occupy
+	 * state space but there is no state to copy from in the
+	 * compacted init_fpstate. The gap tracking will zero these
+	 * states.
+	 *
+	 * The extended features have an all zeroes init state. Thus,
+	 * remove them from 'mask' to zero those features in the user
+	 * buffer instead of retrieving them from init_fpstate.
 	 */
-	if (fpu_state_size_dynamic())
-		mask &= (header.xfeatures | xinit->header.xcomp_bv);
+	mask = header.xfeatures;
 
 	for_each_extended_xfeature(i, mask) {
 		/*
@@ -1151,9 +1150,8 @@ void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
 			pkru.pkru = pkru_val;
 			membuf_write(&to, &pkru, sizeof(pkru));
 		} else {
-			copy_feature(header.xfeatures & BIT_ULL(i), &to,
+			membuf_write(&to,
 				     __raw_xsave_addr(xsave, i),
-				     __raw_xsave_addr(xinit, i),
 				     xstate_sizes[i]);
 		}
 		/*
-- 
2.17.1

[PATCH 2/2] selftests/x86/amx: Add a ptrace test

[Chang S. Bae]

Include a test case to validate the XTILEDATA injection to the target.

Also, it ensures the kernel's ability to copy states between different
XSAVE formats.

Refactor the memcmp() code to be usable for the state validation.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: x86@kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 tools/testing/selftests/x86/amx.c | 108 +++++++++++++++++++++++++++++-
 1 file changed, 105 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/x86/amx.c b/tools/testing/selftests/x86/amx.c
index 625e42901237..d884fd69dd51 100644
--- a/tools/testing/selftests/x86/amx.c
+++ b/tools/testing/selftests/x86/amx.c
@@ -14,8 +14,10 @@
 #include <sys/auxv.h>
 #include <sys/mman.h>
 #include <sys/shm.h>
+#include <sys/ptrace.h>
 #include <sys/syscall.h>
 #include <sys/wait.h>
+#include <sys/uio.h>
 
 #include "../kselftest.h" /* For __cpuid_count() */
 
@@ -583,6 +585,13 @@ static void test_dynamic_state(void)
 	_exit(0);
 }
 
+static inline int __compare_tiledata_state(struct xsave_buffer *xbuf1, struct xsave_buffer *xbuf2)
+{
+	return memcmp(&xbuf1->bytes[xtiledata.xbuf_offset],
+		      &xbuf2->bytes[xtiledata.xbuf_offset],
+		      xtiledata.size);
+}
+
 /*
  * Save current register state and compare it to @xbuf1.'
  *
@@ -599,9 +608,7 @@ static inline bool __validate_tiledata_regs(struct xsave_buffer *xbuf1)
 		fatal_error("failed to allocate XSAVE buffer\n");
 
 	xsave(xbuf2, XFEATURE_MASK_XTILEDATA);
-	ret = memcmp(&xbuf1->bytes[xtiledata.xbuf_offset],
-		     &xbuf2->bytes[xtiledata.xbuf_offset],
-		     xtiledata.size);
+	ret = __compare_tiledata_state(xbuf1, xbuf2);
 
 	free(xbuf2);
 
@@ -826,6 +833,99 @@ static void test_context_switch(void)
 	free(finfo);
 }
 
+/* Ptrace test */
+
+/*
+ * Make sure the ptracee has the expanded kernel buffer on the first
+ * use. Then, initialize the state before performing the state
+ * injection from the ptracer.
+ */
+static inline void ptracee_firstuse_tiledata(void)
+{
+	load_rand_tiledata(stashed_xsave);
+	init_xtiledata();
+}
+
+/*
+ * Ptracer injects the randomized tile data state. It also reads
+ * before and after that, which will execute the kernel's state copy
+ * functions. So, the tester is advised to double-check any emitted
+ * kernel messages.
+ */
+static void ptracer_inject_tiledata(pid_t target)
+{
+	struct xsave_buffer *xbuf;
+	struct iovec iov;
+
+	xbuf = alloc_xbuf();
+	if (!xbuf)
+		fatal_error("unable to allocate XSAVE buffer");
+
+	printf("\tRead the init'ed tiledata via ptrace().\n");
+
+	iov.iov_base = xbuf;
+	iov.iov_len = xbuf_size;
+
+	memset(stashed_xsave, 0, xbuf_size);
+
+	if (ptrace(PTRACE_GETREGSET, target, (uint32_t)NT_X86_XSTATE, &iov))
+		fatal_error("PTRACE_GETREGSET");
+
+	if (!__compare_tiledata_state(stashed_xsave, xbuf))
+		printf("[OK]\tThe init'ed tiledata was read from ptracee.\n");
+	else
+		printf("[FAIL]\tThe init'ed tiledata was not read from ptracee.\n");
+
+	printf("\tInject tiledata via ptrace().\n");
+
+	load_rand_tiledata(xbuf);
+
+	memcpy(&stashed_xsave->bytes[xtiledata.xbuf_offset],
+	       &xbuf->bytes[xtiledata.xbuf_offset],
+	       xtiledata.size);
+
+	if (ptrace(PTRACE_SETREGSET, target, (uint32_t)NT_X86_XSTATE, &iov))
+		fatal_error("PTRACE_SETREGSET");
+
+	if (ptrace(PTRACE_GETREGSET, target, (uint32_t)NT_X86_XSTATE, &iov))
+		fatal_error("PTRACE_GETREGSET");
+
+	if (!__compare_tiledata_state(stashed_xsave, xbuf))
+		printf("[OK]\tTiledata was correctly written to ptracee.\n");
+	else
+		printf("[FAIL]\tTiledata was not correctly written to ptracee.\n");
+}
+
+static void test_ptrace(void)
+{
+	pid_t child;
+	int status;
+
+	child = fork();
+	if (child < 0) {
+		err(1, "fork");
+	} else if (!child) {
+		if (ptrace(PTRACE_TRACEME, 0, NULL, NULL))
+			err(1, "PTRACE_TRACEME");
+
+		ptracee_firstuse_tiledata();
+
+		raise(SIGTRAP);
+		_exit(0);
+	}
+
+	do {
+		wait(&status);
+	} while (WSTOPSIG(status) != SIGTRAP);
+
+	ptracer_inject_tiledata(child);
+
+	ptrace(PTRACE_DETACH, child, NULL, NULL);
+	wait(&status);
+	if (!WIFEXITED(status) || WEXITSTATUS(status))
+		err(1, "ptrace test");
+}
+
 int main(void)
 {
 	/* Check hardware availability at first */
@@ -846,6 +946,8 @@ int main(void)
 	ctxtswtest_config.num_threads = 5;
 	test_context_switch();
 
+	test_ptrace();
+
 	clearhandler(SIGILL);
 	free_stashed_xsave();
 
-- 
2.17.1