4.9-stable review patch. If anyone has any objections, please let me know.
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From: Michael Ellerman mpe@ellerman.id.au
commit ca6d5149d2ad0a8d2f9c28cbe379802260a0a5e0 upstream.
GCC 8 warns about the logic in vr_get/set(), which with -Werror breaks the build:
In function ‘user_regset_copyin’, inlined from ‘vr_set’ at arch/powerpc/kernel/ptrace.c:628:9: include/linux/regset.h:295:4: error: ‘memcpy’ offset [-527, -529] is out of the bounds [0, 16] of object ‘vrsave’ with type ‘union <anonymous>’ [-Werror=array-bounds] arch/powerpc/kernel/ptrace.c: In function ‘vr_set’: arch/powerpc/kernel/ptrace.c:623:5: note: ‘vrsave’ declared here } vrsave;
This has been identified as a regression in GCC, see GCC bug 88273.
However we can avoid the warning and also simplify the logic and make it more robust.
Currently we pass -1 as end_pos to user_regset_copyout(). This says "copy up to the end of the regset".
The definition of the regset is: [REGSET_VMX] = { .core_note_type = NT_PPC_VMX, .n = 34, .size = sizeof(vector128), .align = sizeof(vector128), .active = vr_active, .get = vr_get, .set = vr_set },
The end is calculated as (n * size), ie. 34 * sizeof(vector128).
In vr_get/set() we pass start_pos as 33 * sizeof(vector128), meaning we can copy up to sizeof(vector128) into/out-of vrsave.
The on-stack vrsave is defined as: union { elf_vrreg_t reg; u32 word; } vrsave;
And elf_vrreg_t is: typedef __vector128 elf_vrreg_t;
So there is no bug, but we rely on all those sizes lining up, otherwise we would have a kernel stack exposure/overwrite on our hands.
Rather than relying on that we can pass an explict end_pos based on the sizeof(vrsave). The result should be exactly the same but it's more obviously not over-reading/writing the stack and it avoids the compiler warning.
Reported-by: Meelis Roos mroos@linux.ee Reported-by: Mathieu Malaterre malat@debian.org Cc: stable@vger.kernel.org Tested-by: Mathieu Malaterre malat@debian.org Tested-by: Meelis Roos mroos@linux.ee Signed-off-by: Michael Ellerman mpe@ellerman.id.au Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kernel/ptrace.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
--- a/arch/powerpc/kernel/ptrace.c +++ b/arch/powerpc/kernel/ptrace.c @@ -547,6 +547,7 @@ static int vr_get(struct task_struct *ta /* * Copy out only the low-order word of vrsave. */ + int start, end; union { elf_vrreg_t reg; u32 word; @@ -555,8 +556,10 @@ static int vr_get(struct task_struct *ta
vrsave.word = target->thread.vrsave;
+ start = 33 * sizeof(vector128); + end = start + sizeof(vrsave); ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &vrsave, - 33 * sizeof(vector128), -1); + start, end); }
return ret; @@ -594,6 +597,7 @@ static int vr_set(struct task_struct *ta /* * We use only the first word of vrsave. */ + int start, end; union { elf_vrreg_t reg; u32 word; @@ -602,8 +606,10 @@ static int vr_set(struct task_struct *ta
vrsave.word = target->thread.vrsave;
+ start = 33 * sizeof(vector128); + end = start + sizeof(vrsave); ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &vrsave, - 33 * sizeof(vector128), -1); + start, end); if (!ret) target->thread.vrsave = vrsave.word; }