[PATCH 1/1] x86/sev: Fix potential sign extension in amd_enc_status_change_finish()

[YangWencheng]

When npages (int) is left-shifted by PAGE_SHIFT to calculate the size
in bytes, the operation is performed on a signed integer. If npages is
large enough, this can cause:

  1. Signed integer overflow (undefined behavior per C standard)
  2. Sign extension when the result is promoted to unsigned long for
     the enc_dec_hypercall() parameter

Cast npages to unsigned long before the shift to ensure:
  - The shift operation uses unsigned arithmetic
  - No sign extension occurs during parameter promotion
  - Correct size calculation for large page counts (>2^19 pages on 32-bit)

This is a defensive fix; in practice, npages values are typically small,
but the cast ensures correctness for edge cases and silences potential
compiler warnings with -Wshift-overflow or -Wsign-conversion.

Fixes: ac3f9c9f1b37 (x86/sev: Make enc_dec_hypercall() accept a size instead of npages)
Signed-off-by: YangWencheng <east.moutain.yang@xxxxxxxxx>
---
 arch/x86/mm/mem_encrypt_amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index 2f8c32173972..8134fb731a21 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -306,7 +306,7 @@ static int amd_enc_status_change_finish(unsigned long vaddr, int npages, bool en
 		snp_set_memory_private(vaddr, npages);
 
 	if (!cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT))
-		enc_dec_hypercall(vaddr, npages << PAGE_SHIFT, enc);
+		enc_dec_hypercall(vaddr, (unsigned long)npages << PAGE_SHIFT, enc);
 
 	return 0;
 }
-- 
2.43.0