[PATCH AUTOSEL for 3.18 043/101] arm64: ptrace: Fix VFP register dumping in compat coredumps

[Sasha Levin]

From: Dave Martin <dave.mar...@arm.com>

[ Upstream commit af66b2d88a76574d55e81d712292abd34beb6178 ]

Currently, VFP registers are omitted from coredumps for compat
processes, due to a bug in the REGSET_COMPAT_VFP regset
implementation.

compat_vfp_get() needs to transfer non-contiguous data from
thread_struct.fpsimd_state, and uses put_user() to handle the
offending trailing word (FPSCR).  This fails when copying to a
kernel address (i.e., kbuf && !ubuf), which is what happens when
dumping core.  As a result, the ELF coredump core code silently
omits the NT_ARM_VFP note from the dump.

It would be possible to work around this with additional special
case code for the put_user(), but since user_regset_copyout() is
explicitly designed to handle this scenario it is cleaner to port
the put_user() to a user_regset_copyout() call, which this patch
does.

Signed-off-by: Dave Martin <dave.mar...@arm.com>
Signed-off-by: Will Deacon <will.dea...@arm.com>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
---
 arch/arm64/kernel/ptrace.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8a4ae8e73213..2e845f5c8ab2 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -743,7 +743,7 @@ static int compat_vfp_get(struct task_struct *target,
 {
        struct user_fpsimd_state *uregs;
        compat_ulong_t fpscr;
-       int ret;
+       int ret, vregs_end_pos;
 
        uregs = &target->thread.fpsimd_state.user_fpsimd;
 
@@ -751,13 +751,16 @@ static int compat_vfp_get(struct task_struct *target,
         * The VFP registers are packed into the fpsimd_state, so they all sit
         * nicely together for us. We just need to create the fpscr separately.
         */
-       ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs, 0,
-                                 VFP_STATE_SIZE - sizeof(compat_ulong_t));
+       vregs_end_pos = VFP_STATE_SIZE - sizeof(compat_ulong_t);
+       ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs,
+                                 0, vregs_end_pos);
 
        if (count && !ret) {
                fpscr = (uregs->fpsr & VFP_FPSCR_STAT_MASK) |
                        (uregs->fpcr & VFP_FPSCR_CTRL_MASK);
-               ret = put_user(fpscr, (compat_ulong_t *)ubuf);
+
+               ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &fpscr,
+                                         vregs_end_pos, VFP_STATE_SIZE);
        }
 
        return ret;
-- 
2.15.1