x86/boot: Fix kexec booting failure after SEV early boot support
Subsystems (3)
PATCH 1/1 x86/boot: Fix kexec booting failure after SEV early boot support
Commit 1958b5fc4010 ("x86/boot: Add early boot support when running with SEV active") is causing kexec becomes sometimes unstable even if SEV is not active. kexec reboot won't start a second kernel bypassing BIOS boot process, instead, the system got reset.
That's because, in get_sev_encryption_bit function, we are using 32-bit RIP-relative addressing to read the value of enc_bit, but kexec may alloc the early boot up code to a higher location, which is beyond 32-bit addressing limit. Some garbage will be read and get_sev_encryption_bit will return the wrong value, which leads to wrong memory page flag.
This patch removes the use of enc_bit, as currently, enc_bit's only purpose is to avoid duplicated encryption bit reading, but the overhead of reading encryption bit is so tiny, so no need to cache that.
--- arch/x86/boot/compressed/mem_encrypt.S | 19 ------------------- 1 file changed, 19 deletions(-)
arch/x86/boot/compressed/mem_encrypt.S +0 -19
@@ -25,20 +25,6 @@ ENTRY(get_sev_encryption_bit) push %ebx push %ecx push %edx - push %edi - - /* - * RIP-relative addressing is needed to access the encryption bit - * variable. Since we are running in 32-bit mode we need this call/pop - * sequence to get the proper relative addressing. - */ - call 1f -1: popl %edi - subl $1b, %edi - - movl enc_bit(%edi), %eax - cmpl $0, %eax - jge .Lsev_exit /* Check if running under a hypervisor */ movl $1, %eax
@@ -69,15 +55,12 @@ ENTRY(get_sev_encryption_bit) movl %ebx, %eax andl $0x3f, %eax /* Return the encryption bit location */ - movl %eax, enc_bit(%edi) jmp .Lsev_exit .Lno_sev: xor %eax, %eax - movl %eax, enc_bit(%edi) .Lsev_exit: - pop %edi pop %edx pop %ecx pop %ebx
@@ -113,8 +96,6 @@ ENTRY(set_sev_encryption_mask) ENDPROC(set_sev_encryption_mask) .data -enc_bit: - .int 0xffffffff #ifdef CONFIG_AMD_MEM_ENCRYPT .balign 8
Signature · 1 line
2.17.1
Quoted 12 lines · Kairui Song
Commit
1958b5fc4010("x86/boot: Add early boot support when running with SEV active") is causing kexec becomes sometimes unstable even if SEV is not active. kexec reboot won't start a second kernel bypassing BIOS boot process, instead, the system got reset.That's because, in
get_sev_encryption_bitfunction, we are using 32-bit RIP-relative addressing to read the value ofenc_bit, but kexec may alloc the early boot up code to a higher location, which is beyond 32-bit addressing limit. Some garbage will be read andget_sev_encryption_bitwill return the wrong value, which leads to wrong memory page flag.
Quoted 7 lines
This patch removes the use of
enc_bit, as currently, enc_bit's only purpose is to avoid duplicated encryption bit reading, but the overhead of reading encryption bit is so tiny, so no need to cache that.Fixes 1958b5fc4010 ("x86/boot: Add early boot support when running with SEV active")Suggested-by Borislav Petkov <[email protected]>Signed-off-by Kairui Song <[email protected]>
Quoted 55 lines
--- arch/x86/boot/compressed/mem_encrypt.S | 19 ------------------- 1 file changed, 19 deletions(-)arch/x86/boot/compressed/mem_encrypt.S +0
-19@@ -25,20 +25,6 @@ ENTRY(get_sev_encryption_bit) push %ebx push %ecx push %edx - push %edi - - /* - * RIP-relative addressing is needed to access the encryption bit - * variable. Since we are running in 32-bit mode we need this call/pop - * sequence to get the proper relative addressing. - */ - call 1f -1: popl %edi - subl $1b, %edi - - movl enc_bit(%edi), %eax - cmpl $0, %eax - jge .Lsev_exit /* Check if running under a hypervisor */ movl $1, %eax@@ -69,15 +55,12 @@ ENTRY(get_sev_encryption_bit) movl %ebx, %eax andl $0x3f, %eax /* Return the encryption bit location */ - movl %eax, enc_bit(%edi) jmp .Lsev_exit .Lno_sev: xor %eax, %eax - movl %eax, enc_bit(%edi) .Lsev_exit: - pop %edi pop %edx pop %ecx pop %ebx@@ -113,8 +96,6 @@ ENTRY(set_sev_encryption_mask) ENDPROC(set_sev_encryption_mask) .data -enc_bit: - .int 0xffffffff #ifdef CONFIG_AMD_MEM_ENCRYPT .balign 8
tip-bot for Kairui Song
merge bot[tip:x86/urgent] x86/boot: Fix kexec booting failure in the SEV bit detection code
In reply to Kairui Songx86/boot: Fix kexec booting failure in the SEV bit detection code
Commit-ID: bdec8d7fa55e6f5314ed72e5a0b435d90ff90548 Gitweb: https://git.kernel.org/tip/bdec8d7fa55e6f5314ed72e5a0b435d90ff90548 Author: Kairui Song <[email protected]> AuthorDate: Thu, 27 Sep 2018 20:38:45 +0800 Committer: Borislav Petkov <[email protected]> CommitDate: Thu, 27 Sep 2018 19:35:03 +0200
x86/boot: Fix kexec booting failure in the SEV bit detection code
Commit
1958b5fc4010 ("x86/boot: Add early boot support when running with SEV active")
can occasionally cause system resets when kexec-ing a second kernel even if SEV is not active.
That's because get_sev_encryption_bit() uses 32-bit rIP-relative addressing to read the value of enc_bit - a variable which caches a previously detected encryption bit position - but kexec may allocate the early boot code to a higher location, beyond the 32-bit addressing limit.
In this case, garbage will be read and get_sev_encryption_bit() will return the wrong value, leading to accessing memory with the wrong encryption setting.
Therefore, remove enc_bit, and thus get rid of the need to do 32-bit rIP-relative addressing in the first place.
[ bp: massage commit message heavily. ]
--- arch/x86/boot/compressed/mem_encrypt.S | 19 ------------------- 1 file changed, 19 deletions(-)
arch/x86/boot/compressed/mem_encrypt.S +0 -19
@@ -25,20 +25,6 @@ ENTRY(get_sev_encryption_bit) push %ebx push %ecx push %edx - push %edi - - /* - * RIP-relative addressing is needed to access the encryption bit - * variable. Since we are running in 32-bit mode we need this call/pop - * sequence to get the proper relative addressing. - */ - call 1f -1: popl %edi - subl $1b, %edi - - movl enc_bit(%edi), %eax - cmpl $0, %eax - jge .Lsev_exit /* Check if running under a hypervisor */ movl $1, %eax
@@ -69,15 +55,12 @@ ENTRY(get_sev_encryption_bit) movl %ebx, %eax andl $0x3f, %eax /* Return the encryption bit location */ - movl %eax, enc_bit(%edi) jmp .Lsev_exit .Lno_sev: xor %eax, %eax - movl %eax, enc_bit(%edi) .Lsev_exit: - pop %edi pop %edx pop %ecx pop %ebx
@@ -113,8 +96,6 @@ ENTRY(set_sev_encryption_mask) ENDPROC(set_sev_encryption_mask) .data -enc_bit: - .int 0xffffffff #ifdef CONFIG_AMD_MEM_ENCRYPT .balign 8