[PATCH v2 0/1] MR10232: ntdll: Avoid exposing non-user rsp in normal syscall return path

[Hoshino Lina (＠hoshinolina)]

When rsp is outside the kernel stack, the SIGUSR1 handler considers the context "out of syscall" and reports the real register values to wineserver. rsp pointing into the syscall frame is used for iret returns. Commit 0a5f7a710365 moved this earlier in the return path, creating a window in the regular direct return path where rsp points neither to the kernel stack, nor to the expected user stack. When a thread is interrupted by SIGUSR1 within this window, this exposes a bogus stack pointer to GetThreadContext(), breaking the garbage collector in Unity applications.

Fix the regression by first switching to the real user stack (needed so we don't break things by touching other registers), then, only if an iret path is coming up, switching to the syscall frame pointer. This fixes the regression introduced by 0a5f7a710365, since GetThreadContext() will now observe the proper stack pointer during regular syscall returns.

Further work would be needed to solve other corner cases and ensure that the register context returned by GetThreadContext() behaves exactly as on Windows (with no kernel-mode context info ever returned), but that is a larger/more invasive change.

Fixes: 0a5f7a710365 ("ntdll: Switch to the user stack before restoring the %fs register.")

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=59333

Signed-off-by: Hoshino Lina [e-mail]

--
  v2: ntdll: Avoid exposing non-user rsp in normal syscall return path
 
https://gitlab.winehq.org/wine/wine/-/merge_requests/10232

[PATCH v2 1/1] ntdll: Avoid exposing non-user rsp in normal syscall return path

[Hoshino Lina]

From: Hoshino Lina <[e-mail]>

When rsp is outside the kernel stack, the SIGUSR1 handler considers the
context "out of syscall" and reports the real register values to
wineserver. rsp pointing into the syscall frame is used for iret
returns. Commit 0a5f7a710365 moved this earlier in the return path,
creating a window in the regular direct return path where rsp points
neither to the kernel stack, nor to the expected user stack. When a
thread is interrupted by SIGUSR1 within this window, this exposes a
bogus stack pointer to GetThreadContext(), breaking the garbage
collector in Unity applications.

Fix the regression by first switching to the real user stack (needed so
we don't break things by touching other registers), then, only if an
iret path is coming up, switching to the syscall frame pointer. This
fixes the regression introduced by 0a5f7a710365, since
GetThreadContext() will now observe the proper stack pointer during
regular syscall returns.

Further work would be needed to solve other corner cases and ensure that
the register context returned by GetThreadContext() behaves exactly as
on Windows (with no kernel-mode context info ever returned), but that is
a larger/more invasive change.

Fixes: 0a5f7a710365 ("ntdll: Switch to the user stack before restoring the %fs register.")
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=59333
Signed-off-by: Hoshino Lina <[e-mail]>
---
 dlls/ntdll/unix/signal_x86_64.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/dlls/ntdll/unix/signal_x86_64.c b/dlls/ntdll/unix/signal_x86_64.c
index 0c62e8a1ad4..db0a3972ef9 100644
--- a/dlls/ntdll/unix/signal_x86_64.c
+++ b/dlls/ntdll/unix/signal_x86_64.c
@@ -3172,7 +3172,18 @@ __ASM_GLOBAL_FUNC( __wine_syscall_dispatcher,
                    /* push rbp-based kernel stack cfi */
                    __ASM_CFI(".cfi_remember_state\n\t")
                    __ASM_CFI_CFA_IS_AT2(rcx, 0xa8, 0x01) /* frame->syscall_cfa */
-                   "leaq 0x70(%rcx),%rsp\n\t"      /* %rsp > frame means no longer inside syscall */
+
+                   /* switch to user stack */
+                   /* %rsp outside kernel stack means no longer inside syscall */
+                   "movq 0x88(%rcx),%rsp\n\t"
+
+                   "movl 0xb4(%rcx),%edx\n\t"      /* frame->restore_flags */
+                   "testl $0x1,%edx\n\t"           /* CONTEXT_CONTROL | CONTEXT_INTEGER */
+                   "jz 1f\n\t"
+                   /* Set up rsp for iretq return paths below */
+                   "leaq 0x70(%rcx),%rsp\n\t"
+                   "1:\n\t"
+
 #ifdef __linux__
                    "movw 0x338(%r13),%dx\n"        /* amd64_thread_data()->fs */
                    "testw %dx,%dx\n\t"
-- 
GitLab

https://gitlab.winehq.org/wine/wine/-/merge_requests/10232