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parisc: Fix random data corruption from exception handler

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The current exception handler implementation, which assists when accessing
user space memory, may exhibit random data corruption if the compiler decides
to use a different register than the specified register %r29 (defined in
ASM_EXCEPTIONTABLE_REG) for the error code. If the compiler choose another
register, the fault handler will nevertheless store -EFAULT into %r29 and thus
trash whatever this register is used for.
Looking at the assembly I found that this happens sometimes in emulate_ldd().

To solve the issue, the easiest solution would be if it somehow is
possible to tell the fault handler which register is used to hold the error
code. Using %0 or %1 in the inline assembly is not posssible as it will show
up as e.g. %r29 (with the "%r" prefix), which the GNU assembler can not
convert to an integer.

This patch takes another, better and more flexible approach:
We extend the __ex_table (which is out of the execution path) by one 32-word.
In this word we tell the compiler to insert the assembler instruction
"or %r0,%r0,%reg", where %reg references the register which the compiler
choosed for the error return code.
In case of an access failure, the fault handler finds the __ex_table entry and
can examine the opcode. The used register is encoded in the lowest 5 bits, and
the fault handler can then store -EFAULT into this register.

Since we extend the __ex_table to 3 words we can't use the BUILDTIME_TABLE_SORT
config option any longer.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # v6.0+
This commit is contained in:
Helge Deller
2024-01-20 15:29:27 +01:00
parent 20e08a720c
commit 8b1d723956
8 changed files with 108 additions and 71 deletions
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4
arch/parisc/kernel/cache.c
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@@ -854,7 +854,7 @@ SYSCALL_DEFINE3(cacheflush, unsigned long, addr, unsigned long, bytes,
#endif
" fic,m %3(%4,%0)\n"
"2: sync\n"
ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 2b)
ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 2b, "%1")
: "+r" (start), "+r" (error)
: "r" (end), "r" (dcache_stride), "i" (SR_USER));
}
@@ -869,7 +869,7 @@ SYSCALL_DEFINE3(cacheflush, unsigned long, addr, unsigned long, bytes,
#endif
" fdc,m %3(%4,%0)\n"
"2: sync\n"
ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 2b)
ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 2b, "%1")
: "+r" (start), "+r" (error)
: "r" (end), "r" (icache_stride), "i" (SR_USER));
}