Commit 91f0a0c5 authored by Benjamin Berg's avatar Benjamin Berg Committed by Johannes Berg
Browse files

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: default avatarBenjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240919124511.282088-7-benjamin@sipsolutions.net


Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
parent 77eb31b6

arch/um/kernel/um_arch.c

+2 −4
Original line number Diff line number Diff line
@@ -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;

arch/x86/um/shared/sysdep/stub_32.h

+7 −3
Original line number Diff line number Diff line
@@ -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;

}

arch/x86/um/shared/sysdep/stub_64.h

+5 −3
Original line number Diff line number Diff line
@@ -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;

}