x86, 64bit: Fix a possible wraparound bug in switchover in head_64.S
Subsystems (2)
PATCH v2 1/1 x86, 64bit: Fix a possible wraparound bug in switchover in head_64.S
In head_64.S, a switchover has been used to handle kernel crossing 1G, 512G boundaries.
And commit 8170e6bed465b4b0c7687f93e9948aca4358a33b
x86, 64bit: Use a #PF handler to materialize early mappings on demand
said:
During the switchover in head_64.S, before #PF handler is available, we use three pages to handle kernel crossing 1G, 512G boundaries with sharing page by playing games with page aliasing: the same page is mapped twice in the higher-level tables with appropriate wraparound.
But from the switchover code, when we set up the PUD table:
114 addq $4096, %rdx 115 movq %rdi, %rax 116 shrq $PUD_SHIFT, %rax 117 andl $(PTRS_PER_PUD-1), %eax 118 movq %rdx, (4096+0)(%rbx,%rax,8) 119 movq %rdx, (4096+8)(%rbx,%rax,8)
It seems line 119 has a potential bug there. For example, if the kernel is loaded at physical address 511G+1008M, that is
000000000 111111111 111111000 000000000000000000000
and the kernel _end is 512G+2M, that is
000000001 000000000 000000001 000000000000000000000So in this example, when using the 2nd page to setup PUD (line 114~119), rax is 511.
In line 118, we put rdx which is the address of the PMD page (the 3rd page) into entry 511 of the PUD table. But in line 119, the entry we calculate from (4096+8)(%rbx,%rax,8) has exceeded the PUD page. IMO, the entry in line 119 should be wraparound into entry 0 of the PUD table.
The patch fixes the bug.
---
-v2: make the change simpler and avoid using jmp
--- arch/x86/kernel/head_64.S | 6 ++++-- 1 files changed, 4 insertions(+), 2 deletions(-)
arch/x86/kernel/head_64.S +4 -2
@@ -115,8 +115,10 @@ startup_64: movq %rdi, %rax shrq $PUD_SHIFT, %rax andl $(PTRS_PER_PUD-1), %eax - movq %rdx, (4096+0)(%rbx,%rax,8) - movq %rdx, (4096+8)(%rbx,%rax,8) + movq %rdx, 4096(%rbx,%rax,8) + incl %eax + andl $(PTRS_PER_PUD-1), %eax + movq %rdx, 4096(%rbx,%rax,8) addq $8192, %rbx movq %rdi, %rax
Signature · 1 line
1.7.1
tip-bot for Zhang Yanfei
merge bot[tip:x86/urgent] x86-64, init: Fix a possible wraparound bug in switchover in head_64.S
x86-64, init: Fix a possible wraparound bug in switchover in head_64.S
Commit-ID: e9d0626ed43a41a3fc526d1df06122b0d4eac174 Gitweb: http://git.kernel.org/tip/e9d0626ed43a41a3fc526d1df06122b0d4eac174 Author: Zhang Yanfei <[email protected]> AuthorDate: Tue, 14 May 2013 14:48:58 +0800 Committer: H. Peter Anvin <[email protected]> CommitDate: Tue, 28 May 2013 15:41:59 -0700
x86-64, init: Fix a possible wraparound bug in switchover in head_64.S
In head_64.S, a switchover has been used to handle kernel crossing 1G, 512G boundaries.
And commit 8170e6bed465b4b0c7687f93e9948aca4358a33b
x86, 64bit: Use a #PF handler to materialize early mappings on demand
said:
During the switchover in head_64.S, before #PF handler is available, we use three pages to handle kernel crossing 1G, 512G boundaries with sharing page by playing games with page aliasing: the same page is mapped twice in the higher-level tables with appropriate wraparound.
But from the switchover code, when we set up the PUD table:
114 addq $4096, %rdx 115 movq %rdi, %rax 116 shrq $PUD_SHIFT, %rax 117 andl $(PTRS_PER_PUD-1), %eax 118 movq %rdx, (4096+0)(%rbx,%rax,8) 119 movq %rdx, (4096+8)(%rbx,%rax,8)
It seems line 119 has a potential bug there. For example, if the kernel is loaded at physical address 511G+1008M, that is
000000000 111111111 111111000 000000000000000000000
and the kernel _end is 512G+2M, that is
000000001 000000000 000000001 000000000000000000000So in this example, when using the 2nd page to setup PUD (line 114~119), rax is 511.
In line 118, we put rdx which is the address of the PMD page (the 3rd page) into entry 511 of the PUD table. But in line 119, the entry we calculate from (4096+8)(%rbx,%rax,8) has exceeded the PUD page. IMO, the entry in line 119 should be wraparound into entry 0 of the PUD table.
The patch fixes the bug.
--- arch/x86/kernel/head_64.S | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
arch/x86/kernel/head_64.S +4 -2
@@ -115,8 +115,10 @@ startup_64: movq %rdi, %rax shrq $PUD_SHIFT, %rax andl $(PTRS_PER_PUD-1), %eax - movq %rdx, (4096+0)(%rbx,%rax,8) - movq %rdx, (4096+8)(%rbx,%rax,8) + movq %rdx, 4096(%rbx,%rax,8) + incl %eax + andl $(PTRS_PER_PUD-1), %eax + movq %rdx, 4096(%rbx,%rax,8) addq $8192, %rbx movq %rdi, %rax