From: Benjamin Berg <benjamin.berg@intel.com>
Date: Thu, 19 Sep 2024 12:45:07 +0000 (+0200)
Subject: um: Calculate stub data address relative to stub code
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=91f0a0c5cc5bc863888a936fbd05394c6e284466;p=users%2Fjedix%2Flinux-maple.git

um: Calculate stub data address relative to stub code

Instead of using the current stack pointer, we can also use the current
instruction to calculate where the stub data is. With this the stub data
only needs to be aligned to a full page boundary.

Changing this has the advantage that we do not have a hole in the memory
space above the stub data (which would need to be explicitly cleared).

Another motivation to do this is that with the planned addition of a
SECCOMP based userspace the stack pointer may not be fully trustworthy.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240919124511.282088-7-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---

diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index 99cdf4b2d6482..427024f32b9fe 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -325,10 +325,8 @@ int __init linux_main(int argc, char **argv)
 		add_arg(DEFAULT_COMMAND_LINE_CONSOLE);
 
 	host_task_size = os_get_top_address();
-	/* reserve a few pages for the stubs (taking care of data alignment) */
-	/* align the data portion */
-	BUILD_BUG_ON(!is_power_of_2(STUB_DATA_PAGES));
-	stub_start = (host_task_size - 1) & ~(STUB_DATA_PAGES * PAGE_SIZE - 1);
+	/* reserve a few pages for the stubs */
+	stub_start = host_task_size - STUB_DATA_PAGES * PAGE_SIZE;
 	/* another page for the code portion */
 	stub_start -= PAGE_SIZE;
 	host_task_size = stub_start;
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index 0b44a86dd346e..631a18d0ff441 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -112,10 +112,14 @@ static __always_inline void *get_stub_data(void)
 	unsigned long ret;
 
 	asm volatile (
-		"movl %%esp,%0 ;"
-		"andl %1,%0"
+		"call _here_%=;"
+		"_here_%=:"
+		"popl %0;"
+		"andl %1, %0 ;"
+		"addl %2, %0 ;"
 		: "=a" (ret)
-		: "g" (~(STUB_DATA_PAGES * UM_KERN_PAGE_SIZE - 1)));
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)),
+		  "g" (UM_KERN_PAGE_SIZE));
 
 	return (void *)ret;
 }
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 8e4ff39dcade5..17153dfd780a4 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -117,10 +117,12 @@ static __always_inline void *get_stub_data(void)
 	unsigned long ret;
 
 	asm volatile (
-		"movq %%rsp,%0 ;"
-		"andq %1,%0"
+		"lea 0(%%rip), %0;"
+		"andq %1, %0 ;"
+		"addq %2, %0 ;"
 		: "=a" (ret)
-		: "g" (~(STUB_DATA_PAGES * UM_KERN_PAGE_SIZE - 1)));
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)),
+		  "g" (UM_KERN_PAGE_SIZE));
 
 	return (void *)ret;
 }