[PATCH v2 2/5] powerpc: Allow 4224 bytes of stack expansion for the signal frame
Michael Ellerman
mpe at ellerman.id.au
Fri Jul 24 19:25:25 AEST 2020
We have powerpc specific logic in our page fault handling to decide if
an access to an unmapped address below the stack pointer should expand
the stack VMA.
The code was originally added in 2004 "ported from 2.4". The rough
logic is that the stack is allowed to grow to 1MB with no extra
checking. Over 1MB the access must be within 2048 bytes of the stack
pointer, or be from a user instruction that updates the stack pointer.
The 2048 byte allowance below the stack pointer is there to cover the
288 byte "red zone" as well as the "about 1.5kB" needed by the signal
delivery code.
Unfortunately since then the signal frame has expanded, and is now
4224 bytes on 64-bit kernels with transactional memory enabled. This
means if a process has consumed more than 1MB of stack, and its stack
pointer lies less than 4224 bytes from the next page boundary, signal
delivery will fault when trying to expand the stack and the process
will see a SEGV.
The total size of the signal frame is the size of struct rt_sigframe
(which includes the red zone) plus __SIGNAL_FRAMESIZE (128 bytes on
64-bit).
The 2048 byte allowance was correct until 2008 as the signal frame
was:
struct rt_sigframe {
struct ucontext uc; /* 0 1440 */
/* --- cacheline 11 boundary (1408 bytes) was 32 bytes ago --- */
long unsigned int _unused[2]; /* 1440 16 */
unsigned int tramp[6]; /* 1456 24 */
struct siginfo * pinfo; /* 1480 8 */
void * puc; /* 1488 8 */
struct siginfo info; /* 1496 128 */
/* --- cacheline 12 boundary (1536 bytes) was 88 bytes ago --- */
char abigap[288]; /* 1624 288 */
/* size: 1920, cachelines: 15, members: 7 */
/* padding: 8 */
};
1920 + 128 = 2048
Then in commit ce48b2100785 ("powerpc: Add VSX context save/restore,
ptrace and signal support") (Jul 2008) the signal frame expanded to
2304 bytes:
struct rt_sigframe {
struct ucontext uc; /* 0 1696 */ <--
/* --- cacheline 13 boundary (1664 bytes) was 32 bytes ago --- */
long unsigned int _unused[2]; /* 1696 16 */
unsigned int tramp[6]; /* 1712 24 */
struct siginfo * pinfo; /* 1736 8 */
void * puc; /* 1744 8 */
struct siginfo info; /* 1752 128 */
/* --- cacheline 14 boundary (1792 bytes) was 88 bytes ago --- */
char abigap[288]; /* 1880 288 */
/* size: 2176, cachelines: 17, members: 7 */
/* padding: 8 */
};
2176 + 128 = 2304
At this point we should have been exposed to the bug, though as far as
I know it was never reported. I no longer have a system old enough to
easily test on.
Then in 2010 commit 320b2b8de126 ("mm: keep a guard page below a
grow-down stack segment") caused our stack expansion code to never
trigger, as there was always a VMA found for a write up to PAGE_SIZE
below r1.
That meant the bug was hidden as we continued to expand the signal
frame in commit 2b0a576d15e0 ("powerpc: Add new transactional memory
state to the signal context") (Feb 2013):
struct rt_sigframe {
struct ucontext uc; /* 0 1696 */
/* --- cacheline 13 boundary (1664 bytes) was 32 bytes ago --- */
struct ucontext uc_transact; /* 1696 1696 */ <--
/* --- cacheline 26 boundary (3328 bytes) was 64 bytes ago --- */
long unsigned int _unused[2]; /* 3392 16 */
unsigned int tramp[6]; /* 3408 24 */
struct siginfo * pinfo; /* 3432 8 */
void * puc; /* 3440 8 */
struct siginfo info; /* 3448 128 */
/* --- cacheline 27 boundary (3456 bytes) was 120 bytes ago --- */
char abigap[288]; /* 3576 288 */
/* size: 3872, cachelines: 31, members: 8 */
/* padding: 8 */
/* last cacheline: 32 bytes */
};
3872 + 128 = 4000
And commit 573ebfa6601f ("powerpc: Increase stack redzone for 64-bit
userspace to 512 bytes") (Feb 2014):
struct rt_sigframe {
struct ucontext uc; /* 0 1696 */
/* --- cacheline 13 boundary (1664 bytes) was 32 bytes ago --- */
struct ucontext uc_transact; /* 1696 1696 */
/* --- cacheline 26 boundary (3328 bytes) was 64 bytes ago --- */
long unsigned int _unused[2]; /* 3392 16 */
unsigned int tramp[6]; /* 3408 24 */
struct siginfo * pinfo; /* 3432 8 */
void * puc; /* 3440 8 */
struct siginfo info; /* 3448 128 */
/* --- cacheline 27 boundary (3456 bytes) was 120 bytes ago --- */
char abigap[512]; /* 3576 512 */ <--
/* size: 4096, cachelines: 32, members: 8 */
/* padding: 8 */
};
4096 + 128 = 4224
Then finally in 2017, commit 1be7107fbe18 ("mm: larger stack guard
gap, between vmas") exposed us to the existing bug, because it changed
the stack VMA to be the correct/real size, meaning our stack expansion
code is now triggered.
Fix it by increasing the allowance to 4224 bytes.
Hard-coding 4224 is obviously unsafe against future expansions of the
signal frame in the same way as the existing code. We can't easily use
sizeof() because the signal frame structure is not in a header. We
will either fix that, or rip out all the custom stack expansion
checking logic entirely.
Fixes: ce48b2100785 ("powerpc: Add VSX context save/restore, ptrace and signal support")
Cc: stable at vger.kernel.org # v2.6.27+
Reported-by: Tom Lane <tgl at sss.pgh.pa.us>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
---
v2: Account for the extra 128 bytes of __SIGNAL_FRAMESIZE, making the
total size 4224, as noticed by dja.
See also https://bugzilla.kernel.org/show_bug.cgi?id=205183
---
arch/powerpc/mm/fault.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 641fc5f3d7dd..3ebb1792e636 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -267,6 +267,9 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
return false;
}
+// This comes from 64-bit struct rt_sigframe + __SIGNAL_FRAMESIZE
+#define SIGFRAME_MAX_SIZE (4096 + 128)
+
static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
struct vm_area_struct *vma, unsigned int flags,
bool *must_retry)
@@ -274,7 +277,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
/*
* N.B. The POWER/Open ABI allows programs to access up to
* 288 bytes below the stack pointer.
- * The kernel signal delivery code writes up to about 1.5kB
+ * The kernel signal delivery code writes a bit over 4KB
* below the stack pointer (r1) before decrementing it.
* The exec code can write slightly over 640kB to the stack
* before setting the user r1. Thus we allow the stack to
@@ -299,7 +302,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
* between the last mapped region and the stack will
* expand the stack rather than segfaulting.
*/
- if (address + 2048 >= uregs->gpr[1])
+ if (address + SIGFRAME_MAX_SIZE >= uregs->gpr[1])
return false;
if ((flags & FAULT_FLAG_WRITE) && (flags & FAULT_FLAG_USER) &&
--
2.25.1
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